Check your storage capacity. If your drive has less than 10 GB of free space, the installation file may fail to unpack. App Store Cache Corruption
If iMovie isn't in your "Purchases" history, the App Store may simply tell you it's incompatible. Borrow a Newer Mac: Imovie 10.3.3 Download
This is the headline feature of the 10.3 update. You can edit videos shot in Cinematic mode on an iPhone 13, which allows you to change focus points after recording. With a simple click in the viewer, you can shift focus from one face to another or to specific objects, adding a professional depth-of-field effect to your movies. Check your storage capacity