Hello.

I have to clean the DRC errors for densities.
I was trying to use the tool HIT KIT Utilities but it seems that it does not work properly because it generates some fills but not on top for my design.

Does anybody know the procedure?

Thank you a lot in advance.

It depends if you are drawing the whole layout (for tapeout) or just drawing a function block. Usually the density check is the last step, you can miss them when you draw the function blocks.
When you draw the layout with pad ring, you will do the density check, I think they have the fill layer in the library. You can check all of the cells in all the hit_kit libraries, the fill layer just has layout view, no schematic view.