When trying to add a new category for assignments on Microsoft Classroom, there are a few things to take into account:

1. Ensure that you are logged in with the correct Admin account permissions.
2. Check to see if there is another similar category already listed.
3. Check the spelling of the category name you are entering for accuracy.
4. Verify that the level permissions for the category is correct.
5. Ensure that you have selected the correct school and class year.