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 WorkShop Visual, the Form Layout Editor prevents the creation of invalid links.
See also: