I can add the amc+ app to my tv
which I get from provider Spectrum with my tv plan
but roku won't let me sign in why
Did you add "amc+" or "amc+ on RokuChannel"? They aren't interchangeable. The former is the one you want and it has a sign-in using your provider.
I tried using my provider and it didn't work
In that case, you need to contact amc+ and/or Spectrum and have them figure out why not. It's not a Roku issue - Roku just provides the platform.
I have the exact same issue. The code issued on my Roku Device(s) are not valid.
Any luck with Spectrum?