Enable messages in iCloud option not visible on Mac

I have an iMac and updated to 10.13.5 yesterday but in the Messages app, the option to 'enable messages in iCloud' in not visible.

'About this Mac' is definitely saying version 10.13.5 and when I go to software updates it says 'none available' and I can see that Mac OS High Sierra 10.13.5 Update was installed on 1 June.

But when I go into the Messages app -> Preferences -> Accounts, there is only the option to 'enable this account' (like there was in previous versions of MacOS). There is no option to enable messages in iCloud.

Has anyone any idea what is going on here and what I need to do to fix it?

Thanks.