If you are unable to save a document as .Docx, inspecting the Trust Center should be the first on the list to check followed by the status on the Normal.dotm file in the templafes folder.
Fix 1 — Check Trust Center
1. Open Trust Center
2. Check File Block Settings
3. Check Protected View
4. Restart Word
Close Word completely → reopen → test saving as .docx
Fix 2 — Check Template Path
If Word is pointing to a OneDrive template folder, it may not regenerate Normal.dotm.
1. Open File → Options → Advanced → File Locations
2. Check User Templates.
3. Check to see if it points to OneDrive and change it to: %appdata%\Microsoft\Templates
Close Word completely → reopen → test saving as .docx
Fix 3 — Reset Normal.dotm
1. Close Word
2. Go to: %appdata%\Microsoft\Templates
3. Rename Normal.dotm → Normal.old
Reopen Word to regenerate a clean template → .docx becomes default again.