ERC20-tokens represent a diverse range of crypto assets that don’t have their own blockchain but reside on top of the Ethereum blockchain. ERC-20 is a technical standard emerged to use for all smart contracts on the Ethereum blockchain for token implementations.

To receive an ERC-20 token, you simply have to send it to your Ethereum address. If you haven't noticed already, your ETH receive address is actually the same as your CRO address, so make sure you state clearly to your sender which token you want to receive.

Sending an ERC-20 token is in fact informing the token contract that a transfer is required. To execute this transfer, the contract will require a fraction of Ether called Gas. That’s why you are using ETH as the network fee when sending ERC-20 tokens.

This is also why when your ETH balance = 0, you will not be able to choose to send your ERC-20 tokens from the send coin list.