用tvdiff平行趋势检验，结果no observations [推广有奖]

参考https://mp.weixin.qq.com/s/bJ7kMK6lK21Ra_gYMeiDMA的方法做平行趋势检验代码如下：

xtset id year
generate D = (year - year0 == 0)
global xlist "age tfp le kl sp div er "
tvdiff so2 D $xlist, model(fe) pre(5) post(5) vce(robust) test_tt graph save_graph(mygraph)
产生结果为：
. tvdiff so2 D $xlist, model(fe) pre(4) post(4) vce(robust) test_tt graph save_graph(mygraph)
(0 real changes made)
(0 observations deleted)
       panel variable:  id (unbalanced)
        time variable:  year, 1998 to 2010, but with gaps
                delta:  1 unit
(99,991 missing values generated)
(135,522 missing values generated)
(157,767 missing values generated)
(172,305 missing values generated)
(172,305 missing values generated)
(157,767 missing values generated)
(135,522 missing values generated)
(99,991 missing values generated)
no observations
r(2000);


请问问题出在哪儿，非常感谢！！！