Uniswap Transaction Error – TransferHelper: TRANSFER_FROM_FAILED

I got this to work. I needed to approve the ERC20 token contract to send to the Uniswap V2 router contract, using the standard ERC20 approve(...) function.

My javascript code for this is:

  approveToken() {
    const that = this;
    return new Promise((resolve, reject) => {
      const contract = require('@truffle/contract');
      const transferContract = contract(tokenAbi);
      transferContract.setProvider(that.web3);
      transferContract.at('0xff5254f440816a778851b21d2dfe879b7bad182d').then(function(instance) {
        console.log('we about to approve it');
        return instance.approve( 
           '0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D',
           Web3.utils.toWei("100000000", "ether"),                     
          {
            from: that.account,
            gas: 4000000,
          });
      }).then(function(status) {
        if (status) {
          console.log('erc20 approved');
          return resolve(status);
        }
      }).catch(function(error) {
        console.log(error);
        return reject('transfer.service error');
      });
    });
  }

https://uniswap.org/docs/v2/smart-contracts/router02/ Swaps an exact amount of ETH for as many output tokens as possible, along the route determined by the path. The first element of path must be WETH, the last is the output token, and any intermediate elements represent intermediate pairs to trade through (if, for example, a direct pair does not exist).