Modifying assignment details such as instructions, due dates, or attached resources.
Thanks for the information, Sue.
Based on the link below, you can choose the channel where you'd like notifications for this assignment to post by selecting 'Edit' next to 'Post assignment notifications to this channel'. I believe that's where you get the error, right?
If I may ask, did you assign it to multiple classes? If so, assignments to multiple classes or individual students can only post to the General channel as mentioned on the note from the link:
https://support.microsoft.com/en-us/office/crea...
I hope this helps.
Regards,
Didi