It's obvious it wasn't flashed properly. Assuming its a shingled roof, roof patch stuff can fix some leaks but it can't make water go uphill. The solution undoubtedly involves ripping some shingles off and fixing the problem from the roof deck up. You don't say whether it's an intersection of two roofs, or a roof line abutting a wall. If it's two roofs intersecting, the wet patch stuff might help once you get a day or two where it's practical to apply it. It'll be really ugly, though, unless you're putting it on black shingles, which will make it merely unattractive to the informed eye.

Good luck. Hate finding these things out now.

W-O