variable                    Description
  ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
      id                   unit identifier
      time                 time variable
      y                    dependent variable
      treated              =1 for treated units; =0 for control (never-treated) units
      treated_post         =1 for all observations from treated units after policy intervention; =0 otherwise
      x1, x2, ...          other covariates
    ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

    You must declare your data as panel data before using pcdid:
        xtset id time

    PCDID model with the number of factors determined automatically:
        pcdid y treated treated_post x1 x2

以上是对PCDID命令的介绍。
我的命令是 pcdid y2 treatC did lnGDP lnPGDP
一开始怀疑是缺失数据的问题，将被解释变量y2的缺失值已经删除了，尝试了 pcdid y2 treatC did 依旧报错，有没有大神知道是什么原因呀？