Hello i need help for make mod

Try something like this:

Replace the spear with your custom item, and the potion effect with the potion effect you want. (You can also change the potion particles to true, if you want bubbles around the target.) This procedure will trigger when an entity attacks, check if the item in their main hand is your custom item, then check if they already have a potion effect. If they already have the effect, it removes the effect. If they don't already have the effect, it adds the effect.

Did you set the global trigger to "Entity Attacked?" Also make sure you're using 'source entity' for the first part, not 'event/target entity.' I have tested this on my end. I recommend checking again.

If you're running the procedure using "when living entity is hit with item," you could try doing it like this instead:

I do not know why you are getting an error, as the first example should still work if you are running it through "when living entity is hit with item," but this version should be simpler, if you want to do it this way.

Weird. Try replacing "event/target entity" with "source entity." It is possible they are switched for this trigger. (If this is true, then 'source entity' is the entity the item is hitting, and 'event/target entity' is still the entity holding the item.)

‘Entity’ includes mobs, animals, and players, so it should work for mobs. If you want to this with a command, instead of with the item, you would need to be more specific about which entity you are targeting.