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.