Today I had exactly the same issue, iTunes Version 12.9.4.94 on macOS.
A few hours before I had changed my Apple ID to a new email address. I don't know if that is the reason.
Solved as follows:
- Opened Music app on my iPad
- Tried to add a new album (from Apple Music) to my library
- I got a similar white window but on iPad it was showing new terms and conditions
- I agreed to the new terms, and the album got added
- I returned to my iTunes on Mac, but I still had the issue
- I restarted iTunes
- This time it worked! Could add an album again on Mac.