I am working with a balanced individual-year panel dataset. All the individuals are observed for two and only two years. I am trying to do

Code:
 xtreg y x, fe vce(cluster id)
But I get only the coefficients, the standard errors are not produced.

Things work fine when I use random effects or OLS to estimate the model, so I guess that to cluster the standard error at the group level at least three observations are needed for each group when using within estimator. But why? Or maybe my guess is wrong. Thanks ahead for your explanation.