Hi @AmandaYoh,
Anything listed in the list of "external tools" for assignments has to conform to the Learning Tools Interoperability (LTI) standard so that Canvas can send information about the users to the tool and also receive grades back form the tool. LTI apps have to configured in Canvas and sometimes within the tool itself, either of which may require administrator access and approvals. If you're simply wanting students to go visit an external site/app, but do not expect any login or to get any grades back, you could just make an online type assignment and put a link to the website/app in the assignment description textbox.
Let us know if this helps, or if I've perhaps misunderstood your question.
-Chris
