Transfers from an Ethereum wallet require ETH to pay miner fees. This message means your current account does not have enough ETH. Add enough ETH to your account, then try the transaction again.
For example, you can withdraw ETH from an exchange to this wallet. Select the Ethereum (ERC20) network when withdrawing.
Check the transfer page for the current estimated fee. If you want to set aside ETH for several future transfers, consider keeping around 0.01 ETH for fees. Miner fees are charged by the Ethereum network. imToken does not charge any fees.
Learn more: What is the miner fee?