What are the requirements for national parks?

I'm trying to place a national park here:

But the button to create one with the Naturalist is greyed out:

I know the civopedia lists the requirements for building a national park, but they are either wrong or I'm having a hard time understanding them:

• The tiles must be natural wonders, Mountains or a tile with an Appeal of Charming or better. As you can see the top are both Breathtaking and the bottom ones are part of the Great Barrier Reef wonder
• All four tiles must be owned by the same city. All tiles are within the boarders of Mbwila
• The tiles for a diamond shape. All tiles are connected close to each other in a diamond shape
• No tile can have an improvement or district on it. All tiles are in their initial condition.

So I'm probably missing something. Can someone tell me what's the problem?

Solution 1:

The problem is for sure that your diamond shape is diagonal instead of vertical. (Note that the Spring 2017 update, or an update prior to that, fixed the Civilopedia entry to specify that the diamond must be vertical.)

For this to be a legal configuration for a national park, it would have to include your Niter mine, the two land squares you want, and the rightmost of the water tiles you want to use. However, this is also conditional on your tiles being appealing enough since they are not natural wonders or mountains:

You're probably SOL unless you want to scrap your Niter mine and possibly plant a forest instead (I'm not sure if new-growth forest can be a national park or it's only old-growth (never cleared or planted)).

Frankly I think national parks are either underpowered or too hard to establish.