When user want to set Delegates for a user of a mailbox, the user meet below error.

Step 1:

Step 2:

Step 3:

Error Message:

Solution:

1. Check user mailbox location, local mailbox or office online mailbox.
   
2. Check mailbox location. Method is same as previous step.
   
3. Check GrantSendOnBehalfTo property of this mail box.
   
4. Add user permission.
   

Additional:

Remove user permission.