Cannot join two lines

[Chief Cartographer]

I have two lines I want to join. They are on separate paths. They are not compound. They are not grouped. I used direct select to select the two endpoints, shown in red. I try to join, I get this error. This happens frequently but seemingly randomly and I cannot figure it out.

[Monika Gause]

You don't need to select the endpoints. If you just select the 2 paths, does it then work?

[Chief Cartographer · Accepted solution]

I figured it out! I don't know how it happened, but what appears to be lines are actually polygons. It's as if each line was duplicated, then the end points of each line were joined to each other, forming a polygon that was perfectly stacked on itself. I used the scissors to cut the endpoints of each line, then deleted the duplicate line. Once this was done, I had real lines again, and I was able to make the joins happen without any issue.