Please Select a valid Kerbal Space Program Folder

[Darth Kerman]

Whenever I try to download KSPx64-Patch-1.9.0-To-1.9.1, 'Please Select a valid Kerbal Space Program Folder' message shows up when I select a folder to be downloaded , so I can't update it. I googled this problem and followed some instructions, but nothing was changed. Please answer this if you knnow how to fix it, or good at programming. I've been doing this for 3 months....

[OHara]

That was an error with the upgrader, reported here (link) and since fixed. 
If you download the current version of the 1.9.0 -> 1.9.1 patcher from the KSP store,  it should work.  If you already have a patcher newer than 1 March 2020, and it doesn't work, you might need to download the full 1.9.1

That was for the KSP store; ask again if you use Valve's Steam system, and somebody who knows that system might know the solution.