I am having very similar issues, with a TK flight booked via AC.
I was able to add the booking to the TK mobile app, but also cannot select a seat. (For some reason the app also shows me an option to upgrade to business class, even though it was already a business class seat that I booked.)
I'm not even able to manage the booking on their website. When I enter the PNR and surname (being sure to use the TK PNR beginning with V, not the AC one) I get a popup that says
Dear passenger,
This reservation cannot be viewed due to the sales channel in which it was formed.
You can apply to the channel where you have made your reservation to make a transaction.
And of course when I log in to AC it tells me I have to go to TK's site to manage it.