I wanna use tape to seal some creases on my sail to prevent it from becoming holes. I saw some old sails before people used finda clear tape with lines pattern (not duct tape). What is it called? Thanks.

for repairs you can use clear Mylar tape. just make sure that you cut off any sharp corners so that it doesn't lift off over time as easily.

www.bunnings.com.au/bear-48mm-x-25m-clear-all-weather-tape_p1661582

If it's a big surface area you want to protect, ask for a free stickers from any brand. It won't be transparent but it will look pro

Whitworths chanderly in Wooloongabba has a good range of mylar, dacton and scrim sail repair tapes

Wouldn't you go to a sailmaker there is a few around two in Wynnum that I know they have sticky back sail repair tape.

If its for creases or small (<40mm) tears in mono, then Mylar sail tape should be fine
If patching holes any bigger or on x ply use sail fabric tape.
Any sail manufacturer/repairer will have heaps of off cuts lying around cheap, if you'r not really concerned on colour schemes.