Removing from mask with brush tool fails in Lightroom Classic.

Are you accidentally selecting a slider in the General panel instead of the mask panel?

You need to supply some more info. How does it fail? Do you get an error message or does it simply not do anything when you brush? If it doesn't seem to do anything when you brush it is usually the brush settings (flow/density). ALso, pay attention to what you are doing. If you are intersecting your existing mask with a brush mask for example, it is not always intuitive (but always mathematically correct) what add and substract mean. They add and substract from the mask that you use to intersect with the original mask for example, which is very different from adding and substracting to the original mask so you have to think through what is actually happening.