I ran into this problem (again) last week.

Call #1 to UA 2P line: "Sorry, you have to go through Air Canada to book this seat. We don't have access to their system." (AFAIK the last sentence is a lie as they can easily pull up the reservation as indicated in call #3.)

Call #2 to AC. Very friendly agent but still no success. Naturally first response was to pass the buck: "Try United." After being told of United's response, some clicks are heard, then: "I don't see why this isn't working. This is not a Tango fare. (Also notes 2P status.) Normally I can request seats but it won't even let me put in the request. We just upgraded our computer software. Try again after Jan 23."

Call #3 to UA 2P line. I am transferred to "international desk" (still 2P?) This time I get someone helpful. "Let me see what I can do." Many keystrokes and 2 (internal) calls later: "I tried to request seats but the system would not confirm. Also called Air Canada but they couldn't do it. Cannot understand why as this is not a Tango fare and you are a 2P. Suggest you
try back another time."

Aaargh! Why is it so difficult for a *A carrier to make a seat assignment on a codeshare? I can't believe that it hasn't aroused more ire on FT. Doesn't anyone else use ual.com to book code-share on Air Canada?

Edited to add:

Call #4 to UA ual.com line. "We can't touch it - I'm transferring you to reservations. Disconnect"

Call #5 to 2P account line: "Sorry we don't have access to reservations, only your MP account. I can transfer you to 2P reservations." After voice prompts, I was connected with an agent who said that she was sending a "request" to AC for specific seats. I guess that is the best they can do. Not holding my breath.