I just want to create one more observation. I did this

sysuse auto, clear
count
local count=r(N)
local countplusone=r(N)+1
replace price = 1 in `countplusone'


But I get this error

observation numbers out of range


But when I manually edit the data, then the stata shows me exactly that code that I tried. Then why can't I do the same thing by typing in this code? Is there a way to get around this?