You have tried to set up an invalid attachment in a form.
Valid attachments extend from one edge of a widget to either an edge of another widget or to the containing form.
Further, widgets and forms must be attached in the same coordinate plane: they must be attached either side to side or top and bottom.
For example, it is invalid to attach the left side of a widget to the bottom of a second widget.
Widgets can only be attached together if they are both contained within the same form.
NOTES
In X-Designer, the Form Layout Editor prevents the creation of invalid links.
See also: