Error when applying Heckman two-stage correction

 age           qualificat~n  qual_6        honors        area          sect_1        sect_7        ydate
byage         qual_1        groupqual     homeowner     exper         sect_2        sect_8        monthlywage
female        qual_2        groupq_1      it            exper2        sect_3        sect_9        lnwage
married       qual_3        groupq_2      city_inhab    private       sect_4        overtime
fam_size      qual_4        groupq_3      city_size     temporary     sect_5        condition
child         qual_5        schooling     region        sector        sect_6        lfp

global y "lnwage" // dependent variable
global x "age qual_1 qual_2 qual_3 qual_5 schooling honors city_size exper exper2 private temporary sect_1 sect_2 sect_3 sect_4 sect_5 sect_6 sect_7 sect_8 overtime" // independent variable
global z "age married fam_size child qual_1 qual_2 qual_3 qual_5 schooling honors homeowner it city_size exper exper2 private temporary sect_1 sect_2 sect_3 sect_4 sect_5 sect_6 sect_7 sect_8 overtime" //set of covariates for the labour force participation equation (lfp), where lfp is equal to 0 if housewives, equal to 1 if working people

oaxaca $y $x, by(female) model1(heckman, twostep select(lfp=$z)) model2(heckman, twostep select(lfp=$z))

Model for group 1
Dependent variable never censored because of selection:
model would simplify to OLS regression
r(498);