Trying to make some stencils, what are the lines in the letters? will they print or it’s just showing up on the screen?
To see if they will show in the print you can download the STL, then import it to your slicer, then slice it then switch over to the sliced preview to see if they show up.
To see why they are there, can you share a link to the stencil? Click share next to download.