I'm not sure to understand you right. Please explain more what you are are trying to do.
Is it a drawing you have made yourself with LibreCAD or is it coming from a different system?
Maybe snap problem: be sure to have Free Snap on, that dotted cross icon.
Or are you trying to select a line lying under/within the image? There is the problem that in this case always the image is selected then. This is a bug. It even happens when the image layer is locked. You have to switch it to invisible to prevent it to be selected. You can select the line only by pulling up a window around it. If this is of use depends on the operation you want to do.