Additive painting on a layer mask not working

Just to clarify, are you saying that you can’t use the paint brush in a Layer Mask with white as the foreground colour?

If yes, check that you are using the paint brush and not one of the alternative brushes.

Check the blend mode in both the options bar and for the layer.  e.g. if the layer was set to Screen then you’d see what you describe.

Are you using a mouse or tablet? If tablet, then it could be the preset you have selected.  e.g. Pressure opacity, flow set to very low value.