The command you have executed is used to create a "genesis block" for the first time when the chain is created.
It seems you have already executed the command in the past. Therefore it is throwing an error because genesis block is only initiated once in its lifetime.
You can directly execute the command multichain-cli chain8 to interact with multichain.
If you want to re-create the chain8 from scratch, you need to delete the instance of chain8 which is already present in your machine.
Please execute the following command to delete the older instance and create a new instance:
cd ~/.multichain && sudo rm -rf chain8
Now exit your terminal.
Open a new Terminal and execute the following command:
multichain-util create chain8 (for creating a chain)
multichaind chain8 -deamon (for creating a genesis block)