[R] seemingly unrelated regression

[livia]

Hello,

I am working on the model of seemingly unrelated regression. I came across
with the error message:

Log determinant of residual covariance: NA 
Warning messages:
  NAs generated in: log(x)

I have three linear models which contain 21 explanatory variables and there
are about 300 data points.

Could anyone please give some idea what is wrong with the model?
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