addLiquidityETH : TransferHelper: TRANSFER_FROM_FAILED

Question

I am trying to understand this code. I have tried different variations, but whatever I do, it is not working. I get this Error: VM Exception while processing transaction: reverted with reason string 'TransferHelper: TRANSFER_FROM_FAILED

I have tried mainnet, testnet, but no luck

function openTrading() external payable onlyOwner() {
        require(!tradingOpen,"trading is already open");
        uniswapV2Router = IUniswapV2Router02(0xbe92671bdd1a1062E1A9F3Be618e399Fb5faCAcE);
        _approve(address(this), address(uniswapV2Router), _totalSupply);
        uniswapV2Pair = IUniswapV2Factory(uniswapV2Router.factory()).createPair(address(this), uniswapV2Router.WETH());
        uniswapV2Router.addLiquidityETH{value: msg.value}(address(this),balanceOf(address(this)),0,0,owner(),block.timestamp);
        IERC20(uniswapV2Pair).approve(address(uniswapV2Router), type(uint).max);
        tradingOpen = true;
    }