Cannot backup to iMac and make a clean "reset"

I still have the problem I posted months ago https://discussions.apple.com/thread/250296219

Briefly, contacts I add from my iPhone to Contacts are not synced to iCloud (only "to" and only Contacts)

As the last suggestion I contacted by phone my local Apple service.

They suggested to "reset" the iPhone: backup to my iMac via iTunes and then restore.

It took me some months to decide because I felt it was going to be a headache.

I'm not sure how to make a clean "reset".

First of all I connected the iPhone to the computer and then opened iTunes.

These are my settings:

If I check to manually backup:

"Back Up Now" is greyed out.

So I'm stucked here.

There's an old post talking about this but it has a broken link.

What can I do?

[Image Edited by Moderator to Remove Personal Information]