There are multiple reasons that a national park cannot be built. Some possible reasons are:

- A national park needs to be a vertical diamond.
- All four tiles need to be unimproved.
- All four tiles need to belong to the same city. You can try swapping tiles between the cities.
- The game automatically selects the highest appeal national park. If you want to make a national park on a different location, make the game's suggestion invalid. This can be done by swapping tiles or by building an improvement.