Greetings kokakos,
Welcome to the Apple Support Communities!
I understand that you downloaded iMovie on your iPad for free when you purchased your iPad, but now you cannot download it again for free. It sounds to me like your iPad may be currently using a different Apple ID that initially downloaded iMovie. As the attached article suggests, I would first make sure you are using the same Apple ID to download past purchases. The term purchases also refers to free applications that were downloaded using your Apple ID.
Download past purchases - Apple Support
Sign in with the Apple ID that you used for the original purchase. Use these steps to see which Apple ID you used to download an item.
Cheers,
Joe