Hello! Do you know if its possible add an alert when an user click on submit orders on fulfill order page? the idea is to alert the user if some sales already has an N number of fulfillments

You need to use saveRecord(scriptContext) if you want a dialogue. You can do a normal alert of use N/ui/dialog Module

yeah but, how do I attach the script to the fulfill bulk page?

That page is not scriptable.

thats what i thought! Thank you both for your help!