Hi Iseult,
Thanks for using Microsoft products and posting in the community, I'm glad to offer help.
Generally, when creating/editing an assignment, teachers can click the "Add resources" button and select "Class Notebook" to link a specific page to this assignment.
In your case, may I which step you saw the error? You can try to open Teams via Safari or other web browser on your iPad to see if there will be any difference.
And if there are other teachers in the same team, you can ask them to check whether they have the same problem.
Besides, if the way you add a page of the class notebook to the assignment is different from the above, please describe how you added it, it would be helpful for us to better understand you situation, your understanding will be highly appreciated.
Please feel free to let me know if there is any progress, I'll continue to help you.
Best Regards,
Arck