I find sail tape only works if the sail has been cleaned, i.e. washed of any salt
and dirt and dried. And even then duct tape sticks better, gaffers tape sticks best. (The two are often confused, but gaffer's tape uses a more expensive adhesive
and costs about 2x-4x more. No gummy mess when you try to remove it.)
For a more permanent repair, plain ripstop or sailcloth glued won with AquaSeal or Goop (both flexible urethane adhesives) can do it. Those pretty much match the quality and endurance of a proper patch that has been stitched on. But for any of them--cleaning the sail first is the key to making it work.