civilization-6
I have Saka Horse Archers. I now have the Stirrups tech and have made a Knight. However I do not have the option to upgrade Saka Horse Archers or Horsemen to Knights.
Why not? Am I missing something?
They are light cavalry – but this is not in wikipedia. What am I missing?
In order to get the Niter resource you'll need to remove the farm and build a mine instead.
Different resources(luxury or strategic) need different improvements. For example, to gain sheep you'll need to build a pasture on that tile, to get oil you'll need to build an oil rig, etc.. When your worker is on the tile that has a resource, it'll usually recommend the improvement for you.
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.
Best Answer
The next upgrade for horsemen is the cavalry from the industrial era.
Saka horse archers are actually classified as a ranged unit, not a cavalry unit at all. They also skip the upgrade to crossbowman and upgrade directly to field cannons, meaning they'll be pretty useless during the medieval/renaissance era.
You can check which class a unit belongs to and what it upgrades to on its civilopedia page.