1) Try changing the Formula to 50. Just the number 50, no math. Save your project, then battle test. If that works properly, it was either a problem with your formula (seems unlikely based on your screenshot), or it was working properly but the respective ATK and DEF stats made it evaluate to a negative amount so 0 damage is inflicted.
2) Leave the Formula at 50 and change the Hit Type to "Certain Hit". Save, then test. If that works properly, the enemy probably has some kind of feature or state that reduces the physical or magical damage rates of incoming damage to zero. You can find this on the Enemies tab.
3) Leave it as 50 and Certain Hit. Go to your plugin manager and turn OFF all plugins in your project. Save, then test. If that works properly, one of your plugins (or an incompatibility between them, or the way you are using them) is causing the damage to be evaluated improperly. You can narrow it down by turning off just one plugin at a time and seeing whether the game runs but you get 0 damage - that is your culprit (or at least one of your culprits).