The Canvas Community site is available to any Canvas user in the world who wants to communicate and collaborate with other users. However, in order to participate in the Community and/or access other areas restricted to registered users, you must be logged into your instance of Canvas, first.
So, if you're having issues with logging into the Community (even when logged into Canvas) try the following:
1. Logout and, then log into Canvas, again.
Your Canvas login url will probably be formatted something like https://myinstitution.instructure.com. Then follow the instructions here: How do I log into the Community with my Canvas Account?
2. Submit a Support Ticket
If option #1 doesn't work, then you may have to file a ticket with Canvas. There may be an issue with your Canvas username or password. Check this guide for additional instruction: How do I get help with Canvas?