Hi ItzSlayerM8,
Welcome to the Apple Community, where some of us help each other troubleshoot :)
Wondering which version of iPad you have, and which operating system you are using (iOS Number) ?
It may be that you have many movies in your storage, and so your iPad doesn't; have enough memory to use iMovie.
Apple support have an article online that can help you check your storage:
How to check the storage on your iPhone, iPad, and iPod touch:
https://support.apple.com/en-us/HT201656
If storage seems to be the problem:
you might want to look through the linked articles listed at the bottom of the article.
Please feel free to post here again, and, especially, please reply if you still need help :)
Hope this helps you :)