I am trying to branch to a certificate of completion if a student achieves 70%or better. Want to branch to a failed screen if not. Can someone explain exactly how to do this. Nothing i've tried works. Thanks!

First thing to remember is that anything built in Courselab is going to be very linear.
So if you want to do this then I'd suggest that you don't try to branch, this really needs you to use sequencing. This kind of stuff is tricky so a simpler approach works.
You just make a single end page, it has the items for BOTH the PASS (certificate) and FAIL (the you have failed message)
Just read the score variable and conditionally show the pass or fail items.