Thanks Serge for the reply.
I have pressed the Synchronize With Cloud button and got a green tick (on the main site).
However on the child site, I still have the message:
Errors:
2021-10-07 01:20:26: Cron: Error occurred while updating SpamFireWall local base. Error:
You can get support any time here: https://www.ads-software.com/support/plugin/cleantalk-spam-protect.