Hey there Looncatt,
Thanks for reaching out to Apple Support Communities. I understand you’re having issues syncing your Mac with iCloud. I’ll be happy to help with this.
Go through the following steps and see if iCloud and your Mac sync after taking each step.
Check for updates to your Mac:
How to update the software on your Mac
Also, double check to make sure your device meets the system requirements:
System requirements for iCloud
See if the date and time settings on your Mac are correct:
Set the date and time on your Mac
Make sure that you're signed in to iCloud with the correct Apple ID. On your Mac, choose Apple menu > System Preferences, click Apple ID, then click iCloud.
Open Safari and load a website, like the Apple website to verify your connection. Make sure you’re not using a captive network, a VPN, or using any firewalls or antivirus programs as well.
Open a secure website, like your Apple ID account page, to see if you can access port 443. If the website won't open, contact your Internet service provider for help. Safari needs access to port 443 to update your information in iCloud. Here’s a link to the Apple ID page:
Manage your Apple ID
If the issue continues after taking these steps, go to Apple menu > System Preferences, click Apple ID, then click Overview. Sign out of your account and restart your Mac. Sign back in when you restart and see if this helps at all.
Cheers