Placement of ruhr Valley (can't build on regular plains?)

[Zoll]

So, I'm trying to find a single city in which it's possible to build the Ruhr Valley, but it's proving to be harder than usual. I just don't understand why I can't place it to the right of the industrial zone in this screenshot?

https://steamcommunity.com/sharedfiles/filedetails/?id=1593552849

Anyone care to help me understand?

[Zoll]

It says no suitable location. And the req's are adjacent to an industrial complex and adjacent to river. In my book that mine is a prime spot but nope :S

[Zoll]

And yes, I do have a factory in the indu-zone.

[donald23]

Because there is a mine. You cannot build a district over an improvement. So send a builder, remove the mine and it should be possible then.
Edit after GhostDragon's post: My bad, it's indeed because of the luxury resource.

[GhostDragon]

Because it is a salt mine and you can't place wonders on luxurys

[cerberusiv]

Yes, the salt is blocking building a wonder on that tile. If there was a farm on that tile you could put Ruhr Valley there. I believe that if a tile has a harvestable resource on it you should be able to harvest the resource if it does block a wonder. However only bonus resources are harvestable iirc and salt is a luxury.

[Zoll]

Wow. Thanks everyone for your help! I never knew.