This works fine

$ terraform console > length(list("", "1", "")3

But if you do

locals {  list_of_things = [    "${a_resource.one.id}",    "${a_resource.two.id}",    "${a_resource.three.id}"  ]}

Then this will fail as it's unable to compute the count... unless the items inlist_of_things already exist, when it works. This means you can extend scripts that plan and run successfully but if you then destroy the resources and start from scratch they don't work.

resource "another_resource" "thing" {  count = "${length(local.list_of_things)}"}

Workaround : replace count with a literal count orlocal orvar variable with a literal value.