Hi luthor27,
Thank you for using the Apple Support Communities! We understand you're unable to redownload previously purchased applications from the App Store since reinstalling macOS on your iMac. We're happy to help.
Thanks for the specific details and screenshot, which are helpful. We have a few questions to help isolate further.
-Does this same behavior occur when you try downloading a new application as well? Or, is it just happening when redownloading previously purchased items?
-Is this an issue with all apps you attempt to redownload from your purchase history, or just apps like Numbers, Pages, Keynote, etc.?
-Do you have any issues when using any other Apple services, such as iTunes, Books, etc.?
Give these steps a try to see if they help: If you can't connect to the App Store, iTunes Store, or other Apple services. This includes, connecting your Mac to a different network (Ethernet or different Wi-Fi if available).
Keep us posted by replying back to this post.
Best regards.