Hello there, @sangnan ...
Let's confirm what you already have in Canvas (no need to capitalize all the letters, by the way). First, go to your "People" page of your course, and then do you have any tabs at the top of that page besides the "Everyone" tab? For example, you might have a tab called "Assignment 1" or "Essay Assignment". These are called your "Group Sets". Then, click on one of those "Group Set" tabs, and confirm that you have the four groups created in that particular "Group Set". For example, you might have four groups for your students: "Red Team", "Blue Team", "Green Team", and "Purple Team". If you don't have these things set up as I've described, then you'll need to look at these Guides to get that created:
Once you've confirmed that you have your "Group Set" and the groups within it set up, then you can go to you assignment to make it a group assignment. You should be able to see this interface when you are editing your assignment details:
How do I assign an assignment to a course group? - Instructure Community
When you get to the option of selecting a "Group Set", you will not be creating a "New Group Category" (a.k.a. Group Set). Instead, you will select your "Group Set" that already exists from your "People" page. It would have the same name as one of the tabs on the "People" page..not the names of the teams, for example (Red, Blue, Green, or Purple, for example).
If you are not seeing the option to create a group assignment in the assignment editing screen, can you provide a screen shot or two of what you are seeing so that we can continue to troubleshoot with you? Looking forward to hearing back from you soon!
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