Creating dynamic variables in terraform template file

Question

I am trying to create a template_file block as written below

data "template_file" "abc_routes_uks" {
count    = length(var.do_routes)
template =  file("${path.module}/do_routes_uks.json")
vars     = {
   format("class%d", (count.index))        = values(var.do_routes)[count.index].class
   format("gateway_ip%d", (count.index))   = values(var.do_routes)[count.index].gateway_ip
   format("network%d", (count.index))      = values(var.do_routes)[count.index].network
   format("mtu%d", (count.index))          = values(var.do_routes)[count.index].mtu
   format("route_name%d", (count.index))   = values(var.do_routes)[count.index].route_name
   }
}

The template file block should pass the values to the json file which looks like

"${route_name0}": {
        "abc": "${class0}",
        "pqr": "${gateway_ip0}",
        "xyz": "${network0}",
        "mno": "${mtu0}" 
    },
    "${route_name1}": {
        "abc": "${class1}",
        "pqr": "${gateway_ip1}",
        "xyz": "${network1}",
        "mno": "${mtu1}"
    },
       .....
       .....        

But I am getting the below error

Error: failed to render : <template_file>:8,12-23: Unknown variable; There is no variable named "route_name0". Did you mean "route_name2"?, and 54 other diagnostic(s) │ │ with module.test.data.template_file.abc_routes_uks[2], │ on ......\modules\tf-mce-azurerm-resource\main.tf line 386, in data "template_file" "abc_routes_uks":

Can anyone guide me to solve this issue?