Pandel Data: Within-firm variation using Xtreg. Split variable in two time periods. "No observation"

Hi!

I have a panel data ( firmID and year). I want to do a within-firm variation to analyse different measures.

It should look like this:

Var1 = size in period 1990 - 2000

Var2 = size in period 2000 - 2010

Var3 = cash
i.year* = year dummy control for fixed effects
Y = growth

Y = Var1 Var2 Var3 i.year*, fe i(firmID) vce(cluster firmID)

I got the error: "No observation"

Is that a result of Var1 and Var2?