Hi

I had run a system GMM panel data regression on STATA 14 and I got

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.

Is running system GMM twostep impossible in this case, however my sample is very small ( Number of obs 157)? And how can I solve this problem?