I want to make the operation "Difference" with two tracks.

One of them is a group created with a clon in mosaic: one straight line 1 x 30 column and turned  6º.

The other track is a square that it was  converted  into an object.

If I apply the operation Difference the information bar says: one of the objects is not a track and it can't do the boolean operation

I don't understand why there is an object different than a track.

Thanks in advance.

Fernando