Help, I'm stuck : ERC20: insufficient allowance

Question

My smart contract create a new token when it's deployed, and people can buy or sell this token. The thing is the sell function don't work and I don't know what I did wrong or not understood.

Variables and constructor :

contract Promoter is Owner{

    **PromoterToken public token;**
    uint public lastOwnership;
    uint public rate = 1000000; 
    **address payable public thisContract = payable(address(this));**
    struct Voter{
        bool isRegistered;
        bool hasVoted;
        uint votedFor;
    }
    mapping(address => Voter) Member;
    address[] whitelist;


    **constructor(){
        token = new PromoterToken(100000000000000000000000000, thisContract);
        lastOwnership = block.timestamp;
        setContractPromoter(address(this));
    }**

Buy and Sell functions :

//this one works
function buyPromoter()external payable{
        uint PromoterAmount = msg.value * rate;
        require(msg.value <= 15 ether, "You can't have more than 15% of the supply");
        require(token.balanceOf(msg.sender) + PromoterAmount <= 15000000 * 10 ** 18);
        require (getFundPromoterTokenBalance() >= msg.value, "");
        (bool sent, ) = thisContract.call{value: msg.value}("");
        require(sent, "Failed to send Ether"); 
        (bool transferOk) = token.transfer(msg.sender, (PromoterAmount));
        require(transferOk, "Failed to send PROMOTER");
        if(seeIfMemberExist(msg.sender) == false){
            whitelist.push(msg.sender);
            Member[msg.sender].isRegistered = true;
        }
    }

//stuck on that one
    function sellPromoter(uint amount)external payable{
        require(amount <= token.balanceOf(msg.sender), "you can't sell token you don't own");
        token.approve(thisContract, amount);
        (bool transferOk) = token.transferFrom(msg.sender, thisContract, amount);
        require(transferOk, "Failed to send PROMOTER");
        (bool sent, ) = msg.sender.call{value: amount / rate}("");
        require(sent, "Failed to send Ether");
        if(token.balanceOf(msg.sender) < 1){
            removeMemberOfWhitelist(msg.sender);
        }
    }

Do someone see the issue ?

Answer 1

Before swapping you need to call 'approve' function an example would be. You need to do this with most ERC20 tokens here is a full example: This example shows that you need to send aprove tx wait for it and then send buy or sell one.

const BUSDamountIn = ethers.utils.parseUnits('100', 18);
let amounts = await routerContract.getAmountsOut(BUSDamountIn, [BUSD, WBNB]);
const WBNBamountOutMin = amounts[1].sub(amounts[1].div(10));

console.log(ethers.utils.formatEther(BUSDamountIn));
console.log(ethers.utils.formatEther(WBNBamountOutMin));

const approveTx = await busdContract.approve(
    router,
    BUSDamountIn
);
let reciept = await approveTx.wait();
console.log(reciept);
// call waht you need now
const swapTx = await routerContract.swapExactTokensForTokens(
    BUSDamountIn,
    WBNBamountOutMin,
    [BUSD, WBNB],
    wallet.address,
    Date.now() + 1000 * 60 * 10,
    {gasLimit: 250000}
)

receipt = await swapTx.wait();
console.log(receipt);