As Bill said, epoxy takes weeks to finish curing at temperatures in the 70s.

Each epoxy is a complicated mix of chemicals which may interact with the mix of chemicals
in each varnish, differently.

One of the first kayaks I built more than 20 years ago has varnish on the inside which never cured properly, because in my ignorance, I used whatever marine varnish my local paint store had available.

The manufacturer of your epoxy should know which varnish will be compatible with that epoxy, so it will cure properly.

The first coat of varnish on epoxy less than a month old, always takes longer to cure. Additional coats should cure in 24 hours and be ready for re-coating.

Uncured varnish can be removed easily with steel wool and alcohol.

It's hard to be patient when you're so close to the finish line of your project, but I'd suggest you scrub down the existing varnish with steel wool and alcohol, to remove any uncured finish, and wait for warmer temperatures to apply multipul coats of a compatible varnish. Find out from the epoxy company which varnish they suggest. In the meantime paddle your boat and have fun.

Good luck!

Rob