There are only two ways that I have come across in Europe on mostly solid walls with "crepie":
One is to plaster it all over (and this requires manual labour to plaster it on and then smooth it with sanding) and the other is to sand it. Both require skills so that the plastering or the sanding renders a smooth flat wall and not something off the Sahara desert. In the US, one day in a fit of creativity, I bought something called Lionboard (like a white particle board) and nailed them to the wall and added trim. Painted the remaining textured walls a bright colour and lived by distracting the eye. I am sure other Houzzers will have more ideas given the range of solutions and products in N. America.

You can either plaster over it which requires a professional if you want it to look good, or you can use a special wallpaper over it. The latter is designed to be painted.