Getting "insufficient allowance" error

Question

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "./TokenA.sol";
import "./TokenB.sol";

contract TokenSwap{
    TokenA public tokenA;
    TokenB public tokenB;
    address public admin;
    uint public fees;
    event swap(string from, string to,uint indexed amt);
    //stores the exchange rates of two tokens
    mapping(string=>mapping(string=>uint))exchangeRates; 
    constructor(address _tokenA,address _tokenB,uint256 _exchangeRateAtoB,uint256 _exchangeRateBtoA)
    {
        admin=msg.sender;
        tokenA=TokenA(_tokenA);
        tokenB=TokenB(_tokenB);
        exchangeRates["A"]["B"]=_exchangeRateAtoB;
        exchangeRates["B"]["A"]=_exchangeRateBtoA;
        // approve
        tokenA.approve(address(this),tokenA.totalSupply());
        tokenB.approve(address(this),tokenB.totalSupply());


    }
    modifier onlyAdmin()
    {
        require(msg.sender==admin,"Only admin can call this function");
        _;
    }
    function setPlatformFees(uint256 fee) public onlyAdmin  returns(uint) 
    {
        fees=fee;
        return fees;
    }
    function swapAtoB(uint amount) public 
    {
        require(amount>0,"amount must be greater than zero");
        require(tokenA.balanceOf(msg.sender)>=amount,"Not enough tokens are present");
        uint exchangeRate=exchangeRates["A"]["B"];
        uint exchangeAmtB=(amount*exchangeRate)/100;
        // fees is deducted as defi platform fee
        uint AmtToTransfer= exchangeAmtB - fees;
        require(AmtToTransfer>0,"amt must be greater than zero");
        require(tokenB.balanceOf(address(this))>AmtToTransfer,"Not enough tokenB to make the swap");
        //exchanges
        tokenA.transferFrom(msg.sender,address(this),amount);
        tokenB.approve(msg.sender,AmtToTransfer);
        tokenB.transferFrom(address(this),msg.sender,AmtToTransfer);
        emit swap("tokenA","tokenB",AmtToTransfer);


        
    }
    function swapBtoA(uint amount) public 
    {
        require(amount>0,"amount must be greater than zero");
        require(tokenB.balanceOf(msg.sender)>=amount,"Not enough tokens are present");
        uint exchangeRate=exchangeRates["B"]["A"];
        uint exchangeAmtA=(amount*exchangeRate)/100;
        // fees is deducted as defi platform fee
        uint AmtToTransfer= exchangeAmtA - fees;
        require(AmtToTransfer>0,"amt must be greater than zero");
        require(tokenA.balanceOf(address(this))>AmtToTransfer,"Not enough tokenB to make the swap");
        //exchanges
        tokenB.transferFrom(msg.sender,address(this),amount);
        tokenA.approve(msg.sender,AmtToTransfer);
        tokenA.transferFrom(address(this),msg.sender,AmtToTransfer);
        emit swap("tokenB","tokenA",AmtToTransfer);


        
    }
}

Error while calling swapAtoB

transact to TokenSwap.swapAtoB errored: Error occured: revert.

revert
    The transaction has been reverted to the initial state.
Reason provided by the contract: "ERC20: insufficient allowance".
Debug the transaction to get more information.

Answer 1

I think the contract failed here

tokenA.transferFrom(msg.sender,address(this),amount);

The msg.sender needs to approve the contract as spender. So you need to make a transaction before calling swapAtoB, in which you call tokenA and make approve(contract_address, amount).

Also it's better to rewrite tokenA.transferFrom(address(this),msg.sender,AmtToTransfer); as tokenA.transfer(msg.sender,AmtToTransfer);