Should you receive an error such as "Failed to send transaction error='insufficient funds for gas'" in the BitBoxApp, you probably don't have enough ETH available in your Ethereum account. Please note that the ERC20 token you would like to send and the ETH you reserved for the gas fee must be part of the same Ethereum account. If your ETH are stored in another Ethereum account, send it to the correct one first. Be aware that a transfer like this will require a gas fee in itself.
Gas fees in the Ethereum network can get quite high. You can find an estimation of the required fee in the BitBoxApp as well as on blockchain explorers like this one. If you do not have enough ETH available at the moment, it might be an option to just wait it out until the fees come down again. Of course, if you're in a hurry, you can always send additional ETH to your Ethereum account.