Are you sure you have only one photo selected?
Can you show us a screen capture of the one photo selected?
How are you assigning the keyword to this one photo?
Tried a few ways:
Clicked into Keyword Tag box and typed
Made a Keyword set and selected from the set
Selected an item in the keyword list
Just looking at your screen-clip.
What Keyword are you trying to assign to this one photo selected?
If it is "Genovesa" you can assign it by clicking in the box to the left of the Keyword "Genovesa" when the cursor hovers over. It will then show '1' instead of '0'
If you had 3 photos selected- clicking the box would add "Genovesa" to 3 photos and show '3'
The Keyword list shows that you have 150 photos keyworded with "Galapagos" and the panel above is displaying all keywords attached to the one selected photo (which in this case is one keyword "Galapagos" until you add another!)