I've moved this thread to RGSSx Script Support (Tutorials is not the place to ask for help with custom scripts). Please be sure to post your threads in the correct forum next time. Thank you.
Please provide a link to the blog or forum post where you found the script. Also a screenshot of your Skills tab with the problem skill selected. I'm guessing you just haven't set it up properly.
Also create a new project, don't add ANY custom scripts, and copy and paste your skill over to that new project. See if it works there. If it doesn't, it's not a problem with the script, but with your skill setup. Do OTHER skills cause damage? I suspect it's either your damage formula, or something you've changed on the actors or enemies that's stopping any damage from that skill (and possibly others as well).