Delegating the BTRST tokens in your crypto wallet (e.g. Metamask) to the on-chain voting portal to vote on on-chain proposals is a blockchain transaction that requires a small amount of ETH (~0.001 ETH) for transaction fees (i.e. gas cost). You will need to have that ETH amount in your Metamask wallet in order to do so.
You do not need ETH for voting on snapshot.org for off-chain voting.