Problem
You can't assign tenants to the user.
Solution
Identify and resolve the issue of assigning tenants to the user in Enterprise Hub.
Assign the first role to the user
Note: Make sure the user is assigned at least one role before assigning tenants to them.
In the side menu, click Users.
Edit a user you want to assign tenants
In the Assigned tenants tab, click Assign The First Role.
In the Assign new role window that opens, select the role to be assigned to the user.

Click Select Roles.
Go back to the Assigned tenants tab and click +Assign New Tenant. For more, see the Create users section in the Manage users in Enterprise Hub article.