请问forvalues i=1/500{ use 3.dta, clear xtset id year keep if year==2020 sample 16, count keep id save match_id.dta, replace merge 1:m id using 3.dta gen treat = (_merge == 3) gen period = (year >= 2019) gen did = treat*period reghdfe sale did $xlist ,absorb(id year) vce(cluster id) mat b[`i',1] = _b[did] mat se[`i',1] = _se[did] mat p[`i',1] = 2*ttail(e(df_r), abs(_b[did]/_se[did]))}这段代码里的mat p[`i',1] = 2*ttail(e(df_r), abs(_b[did]/_se[did]))一直说语法出错应该怎么解决呀？