Hi Andrew,
When we want to create an Assignment in Teams, there are 2 ways.
The first one is we go to the Assignment tab in the side bar and click Create, then it will let us select a Class. The other way is to go to the Class team and click Assignment tab in top bar then create an assignment.
It seems like you can’t see the class you want in this page below. I would like to suggest you search the class team name in the search bar. (If this team is not a class team, you can’t create an assignment for it.)
If you still can’t see the class team, can you go to this Team in Teams tab and check whether you can create assignments in it. Moreover, if you are using Teams application, please go to Teams for Web and check the result.
Best Regards,
Ivy