I drew a line between two centers of projected circles.

Although, in the property manager, the endpoint has no relations, as you can see, I can not move the endpoint of the line.

I drew a line between two centers of projected circles.

Although, in the property manager, the endpoint has no relations, as you can see, I can not move the endpoint of the line.

Taking a guess here - the endpoint of the line and the center of the circle are merged. No relationship will show up because the sketcher only treats this as a single point (as in when two lines are connected together) which is fully defined by the circle.

There is a way to detach the line if you check on Tools -> Sketch Settings -> Detach Segment on Drag you'll be able to drag the line and move the endpoint.

You have the line selected, not the end point. The line has no relations, but the end point of the line does. It is coincident with the center point of the circle.