subscribed to AMC+ on amazon how do I watch it on Roku?

If you pay for a service via some platform payment system, then it’s typically the case that you can only watch on that platform.  This is one reason that many people suggest paying the content providers directly and then logging in via each device. 

Sometimes, you can go to the content provider and do a “forgot password” and get access to them.  However, you could break/confuse the linkage from the payment platform to the content provider.