Thank you @KimberlyStockin for including that screenshot. I was able to replicate your problem with some specific steps:
- Discussion set to require users to post first before seeing replies
- Student user has not yet posted to the discussion
- Discussion's until date has passed
When these three things were true, a student user is unable to subscribe to the discussion. An instructor user still can, however.
I realize the first item in my list is contrary to what you reported, but when I changed the discussion so post first was turned off (and left the until date in the past), the student user could subscribe. I even tried changing the course dates so it ended prior to today and concluding the course. In all attempts, the discussions are still subscribable. Finally, I turned off notifications for the course but the subscribe button was no longer grey.
I can think of two options now. One is whether there are user permissions and course settings that are not the same between the instance of Canvas I'm using and yours, and there are lots of options. The other is to contact the Canvas help desk via the Help menu in your Canvas instance. That will get it to the tech support team. It would be great if their vigilance extended to the Canvas Question Forum, alas it does not.
I'm going to mark this reply as a solution. I realize it does not solve your problem, but our colleagues who have a similar critique will also find the best way to pass on the feedback.