Have a few leaks where the carport roof abuts to the main roof. There is a crack
Line about 20 feet. Has no flasher.
The roof is a flat type made of asphalt sheets. Like shingle material.

Do I use the 45 degree alumium flashing I see at the home depot.?
Or is there another method to fill in the fine crack that exists where the
Two roofs meet??

Sounds like they built the carport and did not flash it properly. You need a piece of metal about 6-8" wide metal in 10' lengths, bent at a 90' angle.

The trick is now getting it under the rolled roofing and behind the siding so it will do its job.

You may need a pro and some $ to do it right.

Good luck Roger

I agree with home repair