These "try again later" errors can be irritating since it's a failure of the fundamental handshake between your specific Mac and a cloud of sync servers that Apple operates. Sometimes it's just busy, sometimes your network isn't passing SSL/ encrypted traffic correctly, sometimes your account needs to be fixed on the server side.
You don't have to manage this from your Mac - you can go to https://www.icloud.com/#settings and it will show you the steps to manage it from iOS and from Mac so you can try your change and check on your status from iOS.
If you want to work this to a conclusion on the Mac, dpending on how technical you want to get, interacting with Apple Support is a quick way to narrow these things I'll mention below down. They'll go over a couple things to try and then escalate to engineering if needed.
If it were me, I'd try signing out and in from any device on the same network as the computer. This can remove a portion of your synced iCloud data and need to re-sync it up and down, so I don't do that without thinking about how I use iCloud and know I might wait a while for an iCloud photo library to sync.
If you only have the default 5 GB of storage space, I'd sign out from your Mac, then restart and then try signing back in. Support will really dig in when you can't get signed in in the first place as there's really only four things to fix there.
- Your computer / network / time are off and need to be fixed
- The network isn't set up correctly
- Your account status is locked or needs a fix from Apple
- You aren't following the steps to get logged in