I have a 2004 expedition and the rear climate control is clicking. The fan works well but cannot set temperature. Please help; thank you in advance.

When and where does the "clicking" occur?
When you state that you cannot set temperature, do you mean you can adjust the system but it isn't doing what your requesting,
~~or~~
Do you mean that you cannot actually move the temp setting?
Are there any Diagnostic Trouble Codes (DTCs) from this system?

Try performing an on-board diagnostic test of the system:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-11.html#post1999642

When the key is in on position (not in acc) the clicking is coming from the passenger rear, from inside the plastic housing. Temp stat is setable but rear climate is non-responsive - only cold air. I don't know if any codes are appearing, I usually don't work on autos except my own brakes and oil change. Due to no work coming in I'm having to learn.

Clicking inside the rear A/C housing usually means that a mode door or air blend door is binding.
Remove that plastic housing and see if something is blocking the door movement or if the mode actuator is possibly stripped.
If that isn't your case, you will need someone locally with the appropriate tools to scan the system and run tests to determine what is at fault.