When you encounter a Windows 10 System Restore Error 0x80070091
This is the work around to fix it :
1. Boot into safe mode.
2. Right click on the start button, left click on Command Prompt (Admin) Window will appear with a Command Prompt ending with >
3. takeown /f “C:\Program Files\WindowsApps”
/r /d Y
Press Enter. Wait for it to finish and go back to Command Prompt>
4. icacls “C:\Program Files\WindowsApps”
/grant “%USERDOMAIN%\%USERNAME%”:(F) /t
Press Enter. You will get a message: Successfully processed xxxxx files: Failed to process 0 files
5. rd /s “C:\Program Files\Windows Apps”
Press Enter. It will ask you to confirm. Yes. This will delete
6. Now Reboot your computer and System Restore should work.
More Fix Options you can find here :