Hi!
Welcome to Microsoft Q&A forum.
What you’re describing matches a known issue with Microsoft To Do when it gets stuck in a “lite” mode (missing editing, subtasks, and sidebar/suggestions). This usually happens due to either a bug in recent updates or corruption in cached app data.
First thing to check is the account in To-Do
- In the app, click your profile picture → ensure you’re signed in with the right Microsoft account.
Update the app
- Open Microsoft Store → Library → Get updates.
- Make sure Microsoft To Do is fully updated.
- Restart your PC then test again.
Reset To Do app cache
- Go to Start → Settings → Apps → Installed apps.
- Find Microsoft To Do, click Advanced options.
- Choose Terminate → Reset.
- (Don’t worry, your work are stored in your Microsoft account, which is signed-in .)
If all else fails you need to reinstall To-Do (again your work are saved using the Microsoft account logged in)