Error: Value for undeclared variable

Fix terraform "Error: Value for undeclared variable"

TL;DR: You passed a -var or tfvars value for a variable the root module never declared. Either add a variable block at the root (and pass it through to the child module that actually uses it) or drop the stray value. Also check shell quoting: a space around = or stray quotes becomes part of the variable name.

The error

Error: Value for undeclared variable

A variable named "environment" was assigned on the command line, but the root module does not declare a variable of that name. To use this value, add a "variable" block to the configuration.

Steps

1. Compare the name in the error to your variable blocks. Expected: you spot a typo, or confirm the variable only exists in a child module.
2. If the value belongs to a child module, add a root-level pass-through:

   variable "environment" {
     type    = string
     default = "dev"
   }
   module "webserver_cluster" {
     source      = "./modules/webserver-cluster"
     environment = var.environment
   }

Expected: the root declares it and forwards it.

1. If the value was a mistake (for example passed to terraform init when it was meant as a backend setting), remove that -var. Expected: the error names nothing.
2. On Windows cmd.exe, single quotes are not quoting characters. Use double quotes with escaped inner quotes, or a tfvars file. Expected: the variable name in the error no longer starts with a stray quote.

When this applies

• plan, apply, or (on newer CLI versions) init fails naming a variable "assigned on the command line" that the root module does not declare.
• The variable is declared inside a module but you passed the value at the root.

When it does NOT apply

• "No value for required variable". That means the variable IS declared but has no value. Opposite problem.
• "Variables not allowed". That is about interpolation inside the backend block, not input variables.

Tool and version compatibility

• Terraform CLI 0.12+ through 1.x. Note Terraform 1.15 validates input variables during init more strictly than 1.14, so stray -var flags that used to be ignored at init now fail there.

Why it happens

Command-line and tfvars values land only in the root module. Child module variables are invisible from the CLI; the root must declare a variable and explicitly pass it into each module block. There is no implicit forwarding.

Edge cases and pitfalls

• Terraform Cloud "Terraform Variables" vs "Environment Variables" are different things. Provider-auth values like the OIDC env vars must be environment variables; setting them as Terraform variables produces this error.
• A space in -var "length = 2" makes the name "length " with a trailing space. Terraform takes the name literally.