Hi...
The error is in line (setq en (entnext)), this take the next line...
Example:(defun c:ss (/ entity p1 p2)
(setq entity (car (entsel "\nSelect Line: ")))
(princ "\n")
(princ "Start of line in space is @:")
(princ (setq p1 (cdr (assoc 10 (entget entity)))))
(princ "\nEnd of line in space is @:")
(setq p2 (cdr (assoc 11 (entget entity))))
) ;_defun