I am trying to mold this part, but when I get to tooling split, I get the error message "cannot knit surfaces." I've read a bunch of the other threads but they haven't been helpful to me. Can anyone take a look at my part and see what I'm doing wrong?  I did use the "check" feature and got the message that I had a few "open surfaces" but I'm not sure quite how to fix/change that. Thanks for any help.

Dropbox - red_black_with_runner.SLDPRT