I have 2 variables, investment and cash flow. I also generated a dummy variable called age, which has 3 levels 1 for young,2 for growth and 3 for mature.Now,I tried to regress investment on cash flow with bysort command. Stata displays an error command ,"not sorted ".I think the problem is with the lag operator. I have appended my commands. Can anyone help me what went wrong.

[bysort age : xtreg investment cashflows l.cashflows i.year,fe vce(robust)] .