As long as your reservation on the TK website or app still shows the flight and otherwise looks normal (apart from the seat selection), you should be OK.
But it indeed looks like TK has zeroed out this flight for whichever reason - or the flight is indeed full.
I would suggest to watch your reservation closely over the coming days. I had a similar situation once and the flight was eventually cancelled - although this was during the time of the airport move, ie exceptional circumstances for TK...
Please keep us updated...