Hi,
I am trying to run the command "SFTFE" for the stochastic frontier function with heteroscedastic. When I execute the command "sftfe ln_y ln_x, est(pde) dist(exp) emean(z) usigma(z)", this error message appear

variance(): 3301 subscript invalid
_sftfe_Get_SoL_and_PoSt_ReS(): - function returned error
_sftfe_sf_est_ml(): - function returned error
<istmt>: - function returned error
How can I fix this problem? Any help is highly appreciated

Best regards,
Jo