This issue usually arises on the Continuing Education instance of the UW System Digital Learning Environment Canvas account and impacts users who are taking non-degree continuing education or professional development courses, but which aren't officially enrolled students of the university. Canvas allows Continuing Education students to log in with non-University of Wisconsin credentials, but Zoom does not accept those non-UW credentials.

Instructors on the Continuing Education instance of Canvas should include links to Zoom meetings in the text of Modules, Pages, Assignments or Discussions. They should direct students to access Zoom meetings through those links.

Instructors should not rely on Continuing Education users accessing Zoom meetings through the Zoom integration in the course navigation, as users without UW email addresses will not be able to access it..

If UW institutions hosting Continuing Education courses can issue campus credentials to Continuing Education users and set those users up in Active Directory, this resolves the issue. Doing so allows Continuing Education users to log in to Zoom with Single Sign On, just like other users.

Note: There are often significant educational or administrative reasons explaining why Continuing Education users have not been granted campus credentials. At many institutions, this may not be possible.

A long term solution for this issue is difficult. 

Zoom uses email addresses as unique identifiers for its users, rather than Canvas User ID. When a user attempts to access Zoom through the Canvas integration, Canvas passes the user's default or preferred email address to Zoom. If this email address isn't a UW email address, Zoom can't connect the Canvas request to a user account on the UW System instance, and the the user cannot log in. Changing this behavior would require significantly changing the way that Zoom defines accounts.