Hello KeepImmigrants,
From your post it appears you are having an issue placing items into a sidebar.
We would suggest restarting the Mac. While simple, this can refresh the macOS, apps and connections.
Without knowing what app is being used, and what the side bar for that particular app is capable of, we are going to suggest that you try to isolate the issue a bit further.
Safe mode can help with this. To start in safe mode, begin with the computer powered off. Follow the directions found here, depending on what type of Mac you have: How to use safe mode on your Mac - Apple Support
The computer will take longer to start in safe mode, and the screen may have some distorted video as it starts up. This is normal. Once you are logged in, use the system and test to see if the issue continues.
If everything seems to work as expected, go ahead and restart the computer normally and test again. Starting in safe mode forces the computer to run some disk checks similar to First Aid. This can resolve some issues.
Cheers.