Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hi Richard,
Please check if the issue happens to all users as well as a new created user when assigning tasks.
If the issue only happens to some certain users, not all, I think the affected people may have already been assign the maximum 250 tasks. Therefore, you can't assign more to them. You can see the Planner limitation in this thread: Planner Limitations.
If this is the case, you need to delete some cases (such as closed ones) which have been assign to affected user before and see if it helps.
Regards,
Sky