why cluster doesn't work in vce option?

[Araceli Ramirez]

Hi,

I'm trying to run this code in Stata

iebaltab age internet household_members age_y_member_1 educ_member_1 d12a_1 if age!=., grpvar(treatment) ///
vce(cluster number_school) ///
savexlsx("$table/base_balance.xlsx") replace ftest ///fmissok

However, Stata generate this answer

The cluster variable in vce(cluster number_school) does not exist or is invalid for any
        other reason. See help vce_option for more information.

number_school is a string variable, and it's like "C007". Every observation has a value in number_school, so I don't understand what is wrong with the code, any ideas?