Each ERC20 transaction requires GAS (GWEI) as a transaction fee. That GAS represents of small amount of Ether. That's why having ETH on your balance is required when sending the tokens. Otherwise, you will be unable to finish your transaction.
Why do I need to have Ethereum on my balance if I send tokens?
Tokens are contracts running on Ethreum blockchain. To receive confirmation your transaction should be approved by blockchain and miners. You pay them a small amount as a commission for the operation.
Why do I pay the fee in Eth (GAS), not tokens?
Fees in Ethereum blockchain are calculated in GAS. Unlike Ether, GAS is a separate unit that depends on computing power cost. In turn, it determines the transaction value for miners. When performing a transaction, your ETH in the wallet gets automatically converted into GAS.
What is GAS?
Gas is an internal pricing to run transaction or contract within Ethereum blockchain.
This is the price for computing powers used in order to include your transaction in the block. This is not the matter of information size, so it doesn't matter how much tokens you are sending: 1, 10, 100 or 1000. It's all about the nodes connected to the network.
So how do I send erc20 tokens properly?
You just need to have enough ETH on your balance to make a transaction.