Greetings Mary,
Thanks for visiting the Apple Support Communities! I understand you're unable to edit mail saved in the Drafts folder of your iPhone.
Is this happening with the Mail app? If so, force Mail to close, restart your iPhone, and then test the behavior: How to force an app to close on your iPhone, iPad, or iPod touch - Apple Support & Restart your iPhone - Apple Support
If this is happening with the Gmail app, then I suggest reaching out to their app developers for assistance with the behavior: How to contact an app developer - Apple Support
If further assistance is needed, it would be helpful to know the behavior you experience when attempting to edit an email you have saved in the Drafts folder.
Cheers.