Sometimes a user can't see a persons schedule to book meetings. They do have access as that has not been turned off, generally at Mason. You will not get a denied entry, but their availabilty with Cross Hatch Marks and the message "No Information" when you hover over the calendar entry.
This is a bug in the Outlook Client.
To fix this, you will need to type in the full email address of the person you wish to schedule. You can't type the name, only the full email address will work firstname.lastname@example.org.
Working 1, typed in full email address from the Scheduling Assistant.
Not Working 2, typed in first name on the Meeting Invite and select the name that popped up.