Syncing with icloud paused - Mac needs to cool down

Could you give us more information?

• Do you have “Optimize Storage” checked in Photos’ Settings
• Do you see all your Mac pictures at iCloud.com?
• Have you checked to see if the Apple Account is really really the same on the Mac and on the iPhone and at iCloud.com?
• Where does your Mac's Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive?
• Do you have more than 10% free storage on your Mac?
• Do you have more than 10% free storage on your iCloud account?

If the MacAir thinks it's hot, and you're sure it's not, then it may be a sensor problem. There may be routines out there that will bypass the sensors, but that sounds really bad to me. You may need to take it to Apple.

But first, try these standard things trouble shooting things to narrow things down:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

Let us know what happens…

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac? If so which and when.