Hello Simmum,
I'm sorry, I got sidetracked. I installed Generations, and gave it a little look... then another and another...
The good news is that I have succeeded to place the playground several time, in different worlds. So it is possible.
Here are the different lot icons when in Edit Town mode:
Residential Lots: - a little square with nothing else = unbuilt residential lot
- an outlined house = unoccupied house
- a filled house = occupied house
Community Lots: - little square and bench = unbuilt community lot
- little bench = built community lot
There is an empty 40x40 lot in Twinbrook, indeed. Go to Edit Town and select it with the "arrow tool". On the description panel of this lot, hit the change lot type icon (home/bench icon). Pick up community lot, select "no visitors allowed" or "big park" in the menu, and validate (big V button). You should receive a pop up saying that the lot type has been successfully changed to community / what you picked. And the icon is changed too. Then you can place the playground from lot bin.
If this still doesn't work for you, here are some questions:
1. Have you try to place it in another world (Sunset Valley...)? With success?
2. Was this save game created before the installation of Generations (and the patches)? If yes, you could try in a new save to see if it works.
3. You could also try to place first another community lot from bin (firestation...), then bulldoze it and replace with the playground.