I have a study using HCUP National Inpatient Sample. My commands were all working until I got to hospital characteristics. This is what I get:

. svy, subpop(NINTcat_2): tab HOSP_REGION NAS if YEAR ==2016, column pearson
no observations in subpop() subpopulation
subpop() = 1 indicates observation in subpopulation
subpop() = 0 indicates observation not in subpopulation
r(461);

Thank you.