Hey I ran into this same issue when we booked a trip on Turkish airlines via United airlines using points. Essentially I got the confirmation email from United then I figured I could go to Turkish airlines website to manage the trip to do things like do seat selection and check in. However as you mentioned it was asking to provide an e-ticket number (if I recall correctly it was saying that the number should begin with 9) which I did not see on the itinerary sent to me from United.

I called United first but they wasn't sure how to get that e-ticker number and suggested that I called Turkish airlines directly. Then I called Turkish and they pretty much told me that I have to work with United so I gave up on it at the time.

I gave it another shot the day before within 24 hours of departure to see if that did the trick, but same result. I called Turkish again to see if there is anyway for me to check-in online and they confirmed that I could not and had to do it in person at the airport also advising us to get there early accordingly.

Note that we flew out of Atlanta, per their recommendation we got there about an hour earlier than we originally planned and I am glad that we did because the line was indeed long at the Turkish airlines check-in counter. From the counter they were able to check us in and do seat selection without any issues. Also since we booked through United our known travelers number didn't get attached to the flight so the agent entered that information to ensure we got the pre-TSA on our boarding pass.

I would say give it a shot to call the Chase call center, but if you get the run around just be prepared to go to the airport a bit early to do everything in person.