The program is unable to determine the slope of the orders across the frame.
In this situation the slope is assumed to be 0. The most likely cause of this problem is cosmic-ray contamination. This can be avoided by using the ech_decos1 option/task to clean the frame prior to slope determination.
The order slope is only used when tracing very poor data and can usually be left at the default value of zero.
In extreme cases you can set the slope manually using the FIGARO command SETOBJ:
SETOBJ object=/reduction-file/.MORE.ECHELLE.ORDER_SLOPE value=nn.nn
where nn.nn is the increase in Y-coordinate per X-pixel increment.
Martin Clayton