Monte Carlo Simulation to generate garch process

[Sarah Hussein]

Can anyone tell what's wrong with my simulation:
////////////////////////////////////////the Code:
clear all
program define MonteCarlo, rclass
import excel "C:\Users\altal\OneDrive\Desktop\Udemy\ma1.xls x", sheet("ma1") firstrow

quietly gen time =_n
quietly tsset time

scalar a0 = 0.01
scalar alpha = 0.05
scalar beta = 0.6

gen v=rnormal(0,1)
gen h=rnormal(0,1)
generate e=sqrt(h)*v
replace h=a0+(alpha*(v[_n-1]^2)+ beta)*h[_n-1]



arch y, arch(1/1) garch(1/1)
end

simulate _cons [ARCH]L1.arch [ARCH]L1.garch, reps(500): MonteCarlo

summarize _cons [ARCH]L1.arch [ARCH]L1.garch
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thanks in advance