Today I faced the omitted variable problem but I do not know what is the reason

My code is
Code:
areg dep_var pt wFIRM_SIZE i.yr if GEOGN=="UNITEDS", a(TYPE2)
Array

But when I added the variable "LNGDP" into the regression, it caused the ommited issue in year 2018

Code:
areg dep_var pt wFIRM_SIZE LNGDP i.yr if GEOGN=="UNITEDS", a(TYPE2)
Array

So, I guess it should be from the lack of observation caused by LNGDP for running a regression in 2018, so I check it

But it seems that it is not the case

Code:
count if GEOGN=="UNITEDS" & yr==2018 & LNGDP!=0 & dep_var!=0

1,348
Can I ask what is the reason for this omitted issue, and how to deal with it?