Have you picked black as your player color? It is a known bug that black and white dice do not show well, or at all. You'll need to pick a different color. The color of the dice you roll is determined by your player color. You
can set your player color by clicking the color swatch to the right of
your name across the bottom of the Roll20 window. Change your player color and if you still aren't seeing your dice, report back to us here.