ERC20: insufficient allowance --> Approve function is not working for depositing ERC-20 tokens in a smart contact

I am creating a smart contract to deposit and withdraw USDT token. But I am getting error while depositing tokens saying that "ERC20: insufficient allowance"

  function depositTokens(uint256 amount, string memory symbol) external {
    accountBalances[msg.sender][symbol] += amount;
    ERC20(whitelistedTokens[symbol]).transferFrom(msg.sender, address(this), amount);

I have even tried with IERC20 token too.

 function depositTokens(string memory token,uint _amount) external payable{
       // Check the user's allowance for the specified token.
    uint256 allowance = ERC20(whitelistedTokens[token]).allowance(msg.sender, address(this));
    // If the user's allowance is less than the amount they are trying to deposit, revert.
    if (allowance < _amount) {
        revert("Insufficient allowance");
    // Transfers the amount of USDT/USDC from user to smart contract.
    ERC20(whitelistedTokens[token]).transferFrom(msg.sender, address(this), _amount);

    //Adding collateral amount to user's colateral account balance.
    userCollateralBalance[msg.sender][token] += calculateAmountOfINRDfromUSD(_amount);