If you have an AMC+ account and you want to watch on our AMC+ app, you just need to download it to your device. The main potential issue is that you accidentally go to the Prime Video AMC+ add-on channel. That's a different subscription type.
How to get the AMC+ app
Look for this when you choose the app:
And do not choose either of the ones below―these are both shortcuts to AMC+ within Prime Video Channels