[Limdep Nlogit List] frontier model

[Daniel Ayalew]

I have made an attempt to estimate a truncated-normal, true fixed effects of stocastic frontier production function. The follwing syntax is used: 

SAMPLE ;All$
FRONTIER ;Lhs=y;Rhs=one,x1;Rh2=one;Model=T$
FRONTIER ;Lhs=y;Rhs=one,x1;Rh2=one;FEM;Pds=_GROUPTI;Eff=utfetn;Par$

However, I keep on getting the following error message:

Error 682: Did not find matching prvious nonpanel FRONTIER model.

Any suggestions??

Regards,