Error: Invalid count argument

Fix terraform "Error: Invalid count argument"

TL;DR: Your count expression touches a value that is not known until apply time. Terraform has to decide how many instances to create before it plans anything, so count can only use values known up front. Rewrite the expression to use a variable or local that is known at plan time, or apply the upstream resource first with -target and then plan again.

The error

Error: Invalid count argument

  on main.tf line 39, in resource "aws_iam_role_policy_attachment" "lambda_vpc":
  39:   count = var.vpc_id != null ? 1 : 0

The "count" value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created.

Steps

1. Find what your count depends on. Read the expression and trace every value back to its source. Expected: you find a resource attribute, module output, or data source value that is only known after apply.
2. Introduce a plan-time boolean that mirrors the intent. For example, add variable "enable_vpc" { type = bool } and use count = var.enable_vpc ? 1 : 0, passing the value from your environment or tfvars. Expected: count no longer references anything computed.
3. Alternative when the value genuinely comes from another resource: apply the upstream resource first with terraform apply -target=module.vpc, then run a normal plan. Expected: the second plan sees the value as known and proceeds.
4. Run terraform validate. Expected: Success! The configuration is valid.

When this applies

• plan fails with Invalid count argument and the message says the value depends on resource attributes not known until apply.
• A count (or module count) is derived from another resource's output, like var.vpc_id != null where the VPC is created in the same apply.

When it does NOT apply

• Invalid for_each argument. Same family, different meta-argument; for_each needs static keys, not counts.
• count referencing module outputs that are already known at plan time. That works fine and does not error.

Tool and version compatibility

• Terraform CLI 0.12+ through 1.x. The rule is unchanged across versions. Note the Terraform 1.16 deferred-actions experiment relaxes unknown-at-plan restrictions; on earlier versions the two-step apply is the way out.

Why it happens

Terraform expands count into resource instances before it evaluates anything else. Expansion is a planning step, and planning cannot proceed on a maybe. A value that only exists after the upstream resource is created is a maybe, so Terraform refuses rather than guess.

Edge cases and pitfalls

• Module-level count has the same rule: if a module input is unknown at plan, every count inside the module that touches it fails.
• The -target workaround is a two-step dance, not a fix. Prefer a plan-time boolean variable for anything permanent.
• try() does not help here. This is an expansion error, not an evaluation error.