'invalid group specification - None of the groups defined by pid are treated
I'm trying to do the DID model in Stata using the following panel data. However, the didregress (Y_it) (D_it), group(pid) time(year) command gives me the following error: invalid group specification - None of the groups defined by pid is treated. I also tried xtdidregress (Y_it) (D_it), group(pid) time(year) and set xtset pid year, delta(2) before. Does anyone know what the cause of this problem is? The dataset looks like this image.
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