... booked on Miles and More.

This is frustrating, because I was given the option to select my seats either during the booking process or after my booking was complete. I made the mistake of waiting until the end, and then got this message:

The following problem has occurred:

• Seat reservation is not available for your selected flight(s).

I should have just selected my seats as soon as I was shown a seat map. I didn't do it because I wanted to check with Seatguru first before I just randomly picked any seat.

How do I select my seats now? One of my segments is long-haul (MUC-SIN), so I'm very particular about where I sit.