Hi jeannette299,
Welcome to Apple Support Communities. We understand you're unable to edit and save videos in the Photos app of your iPhone.
Editing your videos is a feature we'd like for you to enjoy. We'll be happy to help.
If you haven't tried yet, we recommend to restart your iPhone. This can help resolve unexpected behavior: Restart your iPhone
We also suggest having enough available storage space on your iPhone: How to check the storage on your iPhone, iPad, and iPod touch
In addition, please ensure that you have the latest version of iOS 14.0.1 installed. This provides software improvements that can help: Update your iPhone, iPad, or iPod touch
We look forward to hearing back from you with the results.
Kind regards.