报错如下：
Warning: Two-step estimated covariance matrix of moments is singular.
  Using a generalized inverse to calculate robust weighting matrix for Hansen test.
  Difference-in-Sargan/Hansen statistics may be negative.
estimates post: matrix has missing values
                 stata():  3598  Stata returned error
         xtabond2_mata():     -  function returned error
                 <istmt>:     -  function returned error
r(3598);