iMovie stuck on validating

Sometimes a corrupt project in the library can keep it from validating. Try opening iMovie in a new library and see if it will validate. Hold down the Option key while opening iMovie and choose a different library or create a new library in which iMovie will open.

If it's a corrupt project you will need to isolate the project and correct the corruption or remove the project from the library. You can create a new library and copy your projects into it one at a time, closing and opening the library each time, until you find the project that contains the corruption.

Here's a link that might be helpful, although not everything discussed there may apply to your situation or be adviseable:

Stuck on "Validating Library" - Apple Community

-- Rich