When you click on an empty seat, nothing happens.
Make sure you are not Spectating/Kibitzing.
You cannot Join or Start a Table if you are Spectating.
To Stop Spectating: press [Leave Table]
The left image shows the empty seats you can take.
If you see something like on the right hand side image, then you are kibitzing and need to leave the table before taking the seat.
DIDO If you have no pre-arranged partner, then you can advertise for a partner;
OR sit opposite an advertising player at a DIDO table
(a single seated player might just have finished a DIDO round, and the table is no longer in the tournament. Hence you cannot sit opposite that player)