stata 动态面板 gmm：rd(因变量) omitted from dgmmiv() because of collinearity

stata 动态面板回归：
结果出现 因变量 omitted from dgmmiv() because of collinearity，搞不动为啥是因变量 omitted，该怎么解决呢！急！！


下面是程序和结果：
. xtabond rd tax govtsub,lags(2) maxldep(3)  endogenous(roe, lag(0,2)) twostep vce(robust)
note: rd omitted from dgmmiv() because of collinearity.
note: L2.rd omitted because of collinearity.
variance–covariance matrix of the two-step estimator is not full rank
    Two-step estimator is not available. One-step estimator is available and
    variance–covariance matrix provides correct coverage.