Dear all, I try to use the command reghdfe to estimate the following fixed effect model:

Array
My stata code is

reghdfe delta_Y delta_X delta_Z if (year==2000|year == 2005|year == 2010) [aweight=population], absorb(city_year) cluster(city_year)

However, the system reports the error message:

(dropped 360 singleton observations)
insufficient observations
r(2001);

I click the link of the error message and convert to the website about Singletons, Cluster-Robust Standard Errors: http://scorreia.com/research/singletons.pdf


I can't fully understand the error message and how should I fix the code above?