iCloud Error: "you can't sign in at this time", after years using it with no issues. Apple says it's because my device is old...

Hi, I bought a brand new MacBook Pro at a computer store, 11 eleven years. The model is a Late-2011, running Mac Os High Sierra (10.13.6), and I've been using iCloud this whole time with no problem. Last week my MacBook stopped syncing through iCloud (my iPhone and iPad continue syncing fine...) and I signed out to see if the issue would be solved. But it didn't work and, since then can't sign in back to iCloud; keeps getting the following error:

I called Apple support several times, followed their suggestions and nothing worked. The last attempt suggested was to reinstall Mac Os High Sierra, first from recovery mode, and later erasing/formatting the hard drive and doing a clean install. While trying to sign in to iCloud after a clean install, I received the error message below:

Last call to support, they told me they had run out of attempts to solve the issue, because my machine is too old. Any ssugestions will be much appreciated. Thanks from Brazil