Hi all,

it has been sorted out. I reached out to the local TK office for assistance. Apparently, their website has a bug which renders the option of "seat selection" unavailable occasionally during the booking process on their website.

1) To solve this, the buyer needs to complete the booking process first and make payment,
2) Then go to manage booking, enter the PNR and surname
3) Once the page loads, just below the header with your PNR, there is an option called "Additional Services". Click on that to enter the seat selection process.

Note: scrolling right down to the passenger preference table and selecting the "seat selection" tab will not work. You have to select "additional services" at the top of the page to enable seat selection.