Cannot buy/update/download any app from Mac App Store

Hi,

I had to reinstall OS in my MacBook Air 2012. First, I installed El Capitan and everything worked fine. I could download/update/buy apps from Mac App Store without any problems. Updated to OS Mojave. I know I could update my MacBook Air to Catalina but I don't want to because many of my Adobe programs will not work (CS 5.5). Since updating to Mojave I cannot download nor buy any apps. When want to download or buy an app, the spinning wheel occurs. I have managed to download only a single app via safe mode.

I logged out from Mac App Store, logged out from iCloud account, rebooted, reinstalled OS, and did all the steps I found on the web. I am able to download/update/buy anything on my iMac/iPad/iPhone with no problems at all. And as I said, I can download/update/buy anything as long as an older OS is installed. There seems to be a problem with Mac App Store on Mojave. Any ideas what can I do more?

Regards