UA most certainly can book TK flights. That doesn't mean they are able to book all TK fares, but they have the ability to book flights, i.e., there should be an interline agreement for UA to sell.

It's likely one of the following:
- Agent was being truthful, and for whatever reason, the UA/TK fares you wanted aren't combinable. Very possible, especially since it seems two different agents said this
- TK has a plating restriction on the fare you were looking for - and UA can't book the fare you want, but can book a full fare (very common with a number of partners these days)