Getting error "Error in plugin @nomiclabs/hardhat-etherscan: The Etherscan API responded with a failure status." while trying to verify smart contract

Question

I successfully deployed my smart contract on Sepolia Testnet. But while I'm trying to verify the contract through hardhat I am getting error. I looked for everywhere for the answer but couldn't find any. Here is the contract:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

import "../node_modules/@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "../node_modules/@openzeppelin/contracts/access/Ownable.sol";

contract RoboPunksNFT is ERC721, Ownable {
    uint256 public mintPrice;
    uint256 public totalSupply;
    uint256 public maxSupply;
    uint256 public maxPerWallet;
    bool public isPublicMintEnabled;
    string internal baseTokenURI;
    address payable public withdrawWallet;
    mapping(address => uint256) public walletMints;

    constructor() payable ERC721("RoboPunks", "RP") {
        mintPrice = 0.02 ether;
        totalSupply = 0;
        maxSupply = 1000;
        maxPerWallet = 3;
        //set withdraw wallet address
    }

    function setIsPublicMintEnabled(bool isPublicMintEnabled_)
        external
        onlyOwner
    {
        isPublicMintEnabled = isPublicMintEnabled_;
    }

    function setBaseTokenURI(string calldata baseTokenURI_) external onlyOwner {
        baseTokenURI = baseTokenURI_;
    }

    function tokenURI(uint256 tokenId_)
        public
        view
        override
        returns (string memory)
    {
        require(_exists(tokenId_), "Token does not exist!");
        return
            string(
                abi.encodePacked(
                    baseTokenURI,
                    Strings.toString(tokenId_),
                    ".json"
                )
            );
    }

    function withdraw() external onlyOwner {
        (bool success, ) = withdrawWallet.call{value: address(this).balance}(
            ""
        );
        require(success, "withdraw failed");
    }

    function mint(uint256 quantity_) public payable {
        require(isPublicMintEnabled, "minting is not enabled");
        require(msg.value == quantity_ * mintPrice, "wrong mint value");
        require(totalSupply + quantity_ <= maxSupply, "nft sold out");
        require(
            walletMints[msg.sender] + quantity_ <= maxPerWallet,
            "excess max wallet"
        );

        for (uint256 i = 0; i < quantity_; i++) {
            uint256 newTokenId = totalSupply + 1;
            totalSupply++;
            _safeMint(msg.sender, newTokenId);
        }
    }
}

Here is hardhat.config.js:

require("@nomicfoundation/hardhat-toolbox");
// require("@nomiclabs/hardhat-etherscan");
// require("@nomicfoundation/hardhat-verify");
require("dotenv").config();
/** @type import('hardhat/config').HardhatUserConfig */

module.exports = {
  solidity: "0.8.17",
  networks: {
    sepolia: {
      url: `https://eth-sepolia.g.alchemy.com/v2/${process.env.ALCHEMY_SEPOLIA_API_KEY}`,
      accounts: [process.env.SEPOLIA_PRIVATE_KEY],
    },
  },
  etherscan: {
    apiKey: process.env.ETHERSCAN_API_KEY,
  },
};

Here is the command I am giving to verify my deployed smart contract: PS D:\BlockChain\Dapps\full-mint-website> npx hardhat verify --network sepolia <contract address> --show-stack-traces

And here is the error I'm getting:

Nothing to compile
Successfully submitted source code for contract
contracts/RoboPunksNFT.sol:RoboPunksNFT at < contract address >
for verification on the block explorer. Waiting for verification result...

Error in plugin @nomiclabs/hardhat-etherscan: The Etherscan API responded with a failure status.
The verification may still succeed but should be checked manually.
Reason: Fail - Unable to verify. Solidity Compilation Error: Source "node_modules/@openzeppelin/contracts/token/ERC721/ERC721.sol" not found: File not found. Searched the following locations: "".

NomicLabsHardhatPluginError: The Etherscan API responded with a failure status.
The verification may still succeed but should be checked manually.
Reason: Fail - Unable to verify. Solidity Compilation Error: Source "node_modules/@openzeppelin/contracts/token/ERC721/ERC721.sol" not found: File not found. Searched the following locations: "".
    at getVerificationStatus (D:\BlockChain\Dapps\full-mint-website\node_modules\@nomiclabs\hardhat-etherscan\src\etherscan\EtherscanService.ts:116:11)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)
    at attemptVerification (D:\BlockChain\Dapps\full-mint-website\node_modules\@nomiclabs\hardhat-etherscan\src\index.ts:503:30)
    at SimpleTaskDefinition.action (D:\BlockChain\Dapps\full-mint-website\node_modules\@nomiclabs\hardhat-etherscan\src\index.ts:791:48)
    at Environment._runTaskDefinition (D:\BlockChain\Dapps\full-mint-website\node_modules\hardhat\src\internal\core\runtime-environment.ts:330:14)
    at Environment.run (D:\BlockChain\Dapps\full-mint-website\node_modules\hardhat\src\internal\core\runtime-environment.ts:163:14)
    at SimpleTaskDefinition.verifySubtask [as action] (D:\BlockChain\Dapps\full-mint-website\node_modules\@nomiclabs\hardhat-etherscan\src\index.ts:324:28)
    at Environment._runTaskDefinition (D:\BlockChain\Dapps\full-mint-website\node_modules\hardhat\src\internal\core\runtime-environment.ts:330:14)
    at Environment.run (D:\BlockChain\Dapps\full-mint-website\node_modules\hardhat\src\internal\core\runtime-environment.ts:163:14)
    at Environment._runTaskDefinition (D:\BlockChain\Dapps\full-mint-website\node_modules\hardhat\src\internal\core\runtime-environment.ts:330:14)

The dir location node_modules/@openzeppelin/contracts/token/ERC721/ERC721.sol is valid. I can't find what wrong I am doing. [N.B. I have tried to verify manually but the bytecodes doesn't seem to match]

Answer 1

After trying many things lastly I solved the problem just by changing

import "../node_modules/@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "../node_modules/@openzeppelin/contracts/access/Ownable.sol";

to

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/access/Ownable.sol";