egen imp=group( i)

. egen exp=group(j)

. ppmlhdfe Trade Dataij,absorb(imp#year exp#year imp#exp) cluster(imp#exp) d nolog
(dropped 12 observations that are either singletons or separated by a fixed effect)
   GLM::init_variables():  3499  remove_collinears() not found
                 <istmt>:     -  function returned error
报错的原因是啥啊？是不是没有安装好ppmlhdfe的安装包？还是命令有问题