Are you sure the object is selected as the Active Object whenever you switch to sculpt mode? It looks like it is based on the outliner, but the way you can know this is if it’s outlined in a lighter orange color in Object mode.
You have a Modifier on your Cube. My guess is that that’s a Subdivision Surface Modifier. That needs to be applied! There is also a warning in the Modifiers Tab:
Just in case somebody looks at this question, because they can’t figure how to click apply. In Blender 2.9 for some odd reason in the modifier tab, the apply button is gone. Click the arrow next to the camera which is the render button to get the option to click apply. Now when you go to sculpt mode the brushes will work and the warning message will disappear.