Join Clips greyed out
I have two clips in mp4 and there are two clips that I want to join together in iMovie. Can I not join clips made by other programs? Sorry - this is the first time I've used iMovie and I followed the instructions on how to join clips and got to this stage, but am not able to because "Join Clips" is greyed out. Is there another free editor that can do what I want to do, or is there some way to get that "Join Clips" to reappear?
Is there a size limit for clips? One of my clips is a it over 3GB long.
I'm on a late 2018 Macbook Pro with 8GB of Memory.
MacBook Pro 13″, macOS 10.14