nothing found where subexp expected for bysort : xtreg with if condition

[Alli Po]

Hi
I am new to STATA. I want to do xtreg with if condition and use tertile for bysort for my unbalanced panel data.

Example :

id	time	S	y	x
1	1	1	10	20
1	2	1	9	50
1	3	1	4	80
2	1	0	5	66
2	2	0	3	71
3	1	1	11	89
4	1	0	15	10
4	2	0	18	35
4	3	0	20	22
4	4	0	9	98
4	5	0	8	100

code:

xtset id time
gen x_lag1=l1.x
egen x_rank=rank(x_lag1), by(time) unique
replace x_rank=x_rank/_n
xtile tertile= -x_rank , n(3)

bysort tertile : asdoc xtreg y x if S==1 , fe vce(robust) nest reset (I also try " bysort tertile (S) : asdoc xtreg y x1 x2 if S==1 , fe vce(robust) nest reset ")

error:
invalid syntax
invalid syntax
invalid syntax
nothing found where subexp expected
r(3000);

Could you please suggest to me how should I do?
Thanks in advance