i have been stuck on this one for a while and cant get past the miter flange part.

it says to select the inside edge draw a small line from there and then do the miter flange which appears it should run all along the inside edge of the U shaped part.

but all i get is'  not suitable for this miter flange or the top edge flips over to make a normal  flange anlong the top edge .

there is a sketch plane created on all the edges ,but it refuses  to make a miter flange.

this is the only tutorial so far i am stuck on.

any ideas what the problem is ?