Why isn't the following ERC721 contract detected on OpenSea?

Question

After implementing the following simple ERC721 contact, then deploying to Rinkby:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;
import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract NFT is ERC721, Ownable {

    using Strings for uint256;
    uint256[] private allTokens;
    uint256 private tokensMinted;
    uint MAX_SUPPLY;

    constructor() ERC721("My-NFT", "NFT") {
        tokensMinted = 0;
        MAX_SUPPLY = 10000;
    }

    function mintNft(address receiver) external onlyOwner returns (uint256) {
        require(tokensMinted < MAX_SUPPLY, "Already at max supply.");
        tokensMinted = tokensMinted + 1;
        allTokens.push(tokensMinted);
        _mint(receiver, tokensMinted);
        return tokensMinted;
    }

    function totalSupply() external view returns (uint256) {
      return tokensMinted;
    }

    function _baseURI() internal pure override returns (string memory) {
        return "https://REDACTED/api/v1/token/";
    }
}

I received the following error from https://testnets.opensea.io/get-listed/step-two

We couldn't find this contract. Please ensure that this is a valid ERC721 or ERC1155 contract deployed on Rinkeby and that you have already minted items on the contract.

The contract is successfully deployed to Rinkby here: https://rinkeby.etherscan.io/address/0xe7129FC868858130BB3dd5B7e7Ea097894ebA0e4

Answer 1

... and that you have already minted items on the contract. Try to mint at least one token and then resubmit your contract.

Answer 2

We faced the same issue with the opensea saying "Please ensure that this is a valid ERC721 or ERC1155 contract deployed on Rinkeby and that you have already minted items on the contract". Try to re deploy the smart contract again and try