State-time fixed effects in Staggered Difference-in-Difference

Question

I was trying to implement state-year fixed effects in R (did package) but the code kept running.

out1 <- att_gt(yname  = "y", 
               tname   = "year",
               idname  = "id_county",
               gname   = "p_year",
               #xformla = ~1,
               xformla = ~ factor(time)*factor(year),
               control_group = "notyettreated",
               panel = T,
               clustervars = "id_county",
               est_method = "dr",
               data    = da)

Then, I read this paper and found that state-year fixed effect is not possible to implemented in the staggered difference-in-difference model.

Does anyone know why?