Hi,
I wanted to estimate a random effects Heckman sample selection model in which I constrain the level 1 / within variance to zero using xtheckman. However, I have a hard time figuring out how to specify the constraint. I attempted using

cons def 1 _b[/:c_2_1] = 0 and

cons def 2 [/]c_2_1 = 0

but Stata returns an error saying that the constraint returned a code 111.

Does anyone know how to implement such constraint?

Best
Kristian