spdep lagsarlm regression error

It produces the following error.
Error: length(listw$neighbours) == n is not TRUE

Do you have NA values in your dataframe 'mex'? The problem may be that
using the option 'na.action=na.omit', lagsarlm() subsets the listw
object, then the impact() function is waiting for the listw object of
the subset. So, you may want to try to subset your W object, just before
calling impact().
Also I recommend that you check that your data are in the same order
that your listw object:

identical(attr(W, 'region.id'), rownames(sar.mex$X))
identical(attr(W, 'region.id'), names(sar.mex$y))