Hi everyone,
I am running a fixed-effects model with robust standard errors and I'd like to test for within-group serial correlation. The relevant code is the following:

xtset country_num year
xtreg score second_period third_period, fe vce(robust)

I read that I should use xtserial for the test, but when I try to run the following code: xtserial score, I get the error message "no observations". I don't know why I get it and how I could avoid it.
Sorry if this sounds trivial and thanks in advance!