It looks like the road you built is too close to the mine site.

If you remove the road that's right next to the cursor, you should be able to build the mine there.
It has nothing to do with the geologist signs themselves.

I've done the same thing, built a road up to a potential mine area and flag it, then can't build mines as easily;
but it's the road taking up space. Buildings need more than just a single 'flag' of area to be built there.

Also it looks like you have two iron mines being built in that area already.
A helpful tip is that an iron mine can reach iron a little bit away from it, not just the iron it's directly on top of.
So you don't need a mine on every geologist sign to reach all the iron.

You can test this too, by having a mine running for a long time until the materials are exhausted, then send a geologist and see how much of the area around the mine has been spent of resources.