向各位求助，谢谢
研究某省内各城市的数据，给城市分组（使用命令如下）之后
gen group1=""
. replace group1="first" if city_id ==3
variable group1 was str1 now str5
(72 real changes made)

. replace group1="third" if city_id ==2 | city_id ==4 | city_
> id ==6 | city_id ==10 | city_id ==11 | city_id ==12 | city_
> id ==14 | city_id ==15
(576 real changes made)

. replace group1="forth" if city_id ==1 | city_id==5 | city_i
> d==7 | city_id==8 | city_id==9 | city_id==13 | city_id==16


再进行面板回归显示
xtreg lnReinv lnHP L.LnSales L.Finance Loan  i.month, fe if group1==third
option if not allowed

请问应该如何解决，谢谢各位