Cannot assign Zoho Assist app to user

Cannot assign Zoho Assist app to user

Recently, I cannot add user to Zoho Assist app.  Wheneven I assign, the screen just shown "Error occurred".  

Actually, it's not just assigning Zoho Assist app failed.  When I change department of a user, the same "Error occurred" shown. 

Any of you have the same issue?

Thanks

Raymond