You can only pick a seat if there is a seat available to pick from. At this time, there is no seat available to select

Of the few seats in the cabin, those that the OP would be able to select from are already taken

There are only 28 seats in the Premium Economy section. The 1st 2 rows are blocked -except for those holding emerald status and the seats in the last 2 rows are already occupied

How is it strange that selection of a specific seat is therefore unavailable?