1
Open the Settings App.
Go to Apps - Installed Apps.
Scroll down to find the Paint app.
Select that app, then choose Advanced Options, then run the Repair operation, and also the Reset oepration.
Check if that resolves the problem with that app.
2
If that does not resolve the problem, uninstall the Paint app, then restart (not shut down) your PC and install the latest version of the Paint app from the store.