If you've already been prompted by the application, iMovie in this case, to grant it permissions via a pop-up window, regardless of whether you've accepted or declined, the permission can be accessed and changed in the Settings app on your device. Depending on what version of iPadOS you're running, this menu will be in slightly different places, but in iPadOS 18, the menu can be found near the bottom of the main menu. It should be labeled something like "Apps." From there, you can choose the specific app you would like to change the permissions of, and you are presented with toggles for each permission. Ex. Location Services, Camera, Microphone, etc. From there, you can make any alterations you need to.
Alternatively, if the app itself is experiencing issues prohibiting its use, insure that the application is up to date in the App Store, check for any iPad software updates, force quit the app, (Swipe up on it) and if all else fails, restart your device. If the issue persists after these troubleshooting methods, I would advise that you consult an Official Apple Support representative.
I hope this helps!