Hi,

I am trying to estimate this gravity equation:

ppml trade PAIR_FE1-PAIR_FE$NTij_1 EXPORTER_TIME_FE* IMPORTER_TIME_FE1-IMPORTER_TIME_FE$NT_yr RTA, noconst


however, I have this error: "Warning: variance matrix is nonsymmetric or highly singular"

What does it mean? and how can I correct it?

Thank you!