Do you mean the admin cannot open his shifts in Microsoft Teams?
I did a test in my environment and here is our steps for your reference:
1) One user is Teams admin and he can create the shift for himself and open it like the following picture:
2)Other Teams user now can see his shifts in Teams client
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.