Did you check the making surface of the connector to the toilet before you coupled the line to it?

I once paid extra for a toilet flapper rebuild kit with the new seat because it was still leaking after I replaced the old one. It turns out there was just a hard water deposit stuck there that caused the new flapper to not be able to seal. I picked it off with a fingernail and cleaned it with white vinegar and it worked perfectly.

A quick scrub with a descaling cleaner like lime-away or CLR on a soft cloth could remove a nearly invisible deposit that is preventing a watertight seal.