Stroke a Path. Simple, right. Naaaaa.

I can reproduce the issue if I select a Layer on which one cannot paint (see screenshot). 

There are one or two gotyas regarding that process.  The most obvious being that you have to have a raster selected, and the current forground colour needs to differ from the document background colour so you can actually see the stroke.  You might need to check brush and layer blend modes if things do not seem to be working as they should.

The second is that hitting the stroke icon at the bottom of the Paths panel will trigger tha last tool that was used to stroke a path, so if you are not sure, make the first stroke by right clicking the path, choose stroke, and if it does not say Brush use the drop down to select the brush.  You can safely use the icon after that.

Simulate Pressure will make the stroke as if drawn with a tablet and stylus using a Pressure affects size preset.  I use this a lot as it enables accurate brush stroke placement.

Another wee gotya is that if you have multiple paths on the same path and bo points are active, then stroking will cause all of the path components to be stroked.  If you have active points on one or more components (like below) thhen just that component will be stroked.

If you already knew most of this stuff them my apologies. 

Could you please post screenshots (https://www.take-a-screenshot.org) with the pertinent Panels (Toolbar, Layers, Paths, Options Bar, …) visible? 

Is one paintable Layer selected?