I'm trying to sign in to iCloud in my macOs High Sierra with my Apple ID but doesn't work.

Your footer here shows macOS Big Sur, so I’ll assume this is a different Mac.

Reboot the problem Mac, and try again.

Reboot into Safe Mode and try again.

If there’s a VPN client app or add-on security app installed, disable and remove all that per the vendors’ instructions, restart, and try again.

If the Mac supports something newer than macOS 10.13, consider an upgrade.

nelia68 Said:

"Why can't I sign in to iCloud with my Apple ID ?"

------

Troubleshooting iCloud Login:

Is this a Used Device?

If you are unable to log into your Apple ID on a used device, then it seems you have been scammed. Though a full-assumption on my part.

So, what to do:

Have the Seller Sign Out of it: 

This is a theft deterrent. Only the person currently signed in to the iPad can sign out of it. So, have them sign out of iCloud on it. If need be, send it back to them, having then sign out of it. This is the site they should have used prior to handing it over to you: What to do Before you Sell, Give Away, or Trade in your iPhone, iPad, or iPod touch - Apple Support