When sculpting cloth nothing happens, object turns dark

I’m new to Blender and I’m following Blender Guru’s tutorial on YouTube (https://www.youtube.com/watch?v=HlrgFS8AMMU at about 11:59).

So far everything has worked according to plan. First I added a cloth physics to the couch cushion with 5 pressure and 0 gravity. Then I tried out the cloth sculpting brush on a flat plane to make sure it works, which it did. I selected my object (couch cushion) and moved to sculpt mode. The object turns dark grey, and sometimes I get a warning that says "object has non-uniform scale, sculpting may be unpredictable." When I use the brush and try the space stroke method or line as the tutorial suggests, nothing happens. I checked my settings to make sure they look the same as they do in the tutorial, and they are.
I have no idea why it is acting up.
Any help would be great, thanks.