Hi all.

Not sure whether I should ask this question separately or use the previous question since it's kind of related but not really.  So the issue I have as you can see from the picture, I'm trying to select the bottom line as a guideline and it's not letting me.  It gave me the failed to complete error.  I don't think it's due to non-intersecting line cuz I checked and checked and don't see one.  Perhaps I miss it?

If I don't use the bottom guideline, the model looks weird after I extrude cut the square part.  Any suggestions on what I miss?  Getting very irritated.  Please help?  Many thanks again, everyone.