That is plumbing solder. That's a first on me (plumbing flux in a plastic container).
A. Made by Rectorseal.
B. Mentions copper and brass fittings. Not many of those in electronics.
C. Mentions usable on potable water systems.
DrAzzy mentioned one source for soldering flux, Here is a link to another
https://www.allelectronics.com/item/sp-44/soldering-paste-flux-50-gram/1.html
Amazon has a lot of stuff, but sometimes it is hard to know what you are getting. If you get solder flux from an electronics shop it is more likely to be for electronics. Just saying.