How to fix Error: Reference to undeclared module in Terraform

Check that you have a module block declaring the module you're trying to reference. For example, if referencing module.networking, you must have declared it:

module "networking" {
  source = "./modules/networking"
  # ... other configuration
}

Ensure the label (name after module) matches exactly what you're referencing elsewhere.

Module names are case-sensitive in Terraform. If your declaration uses module "Networking" but you reference module.networking, it won't work.

Double-check:
- Spelling of the module name
- Capitalization
- Special characters

The label between module "" and the reference must match exactly.

If you have a child module that needs to reference another module's output, you must pass it through the parent (root) module:

In root main.tf:

module "networking" {
  source = "./modules/networking"
}

module "security" {
  source          = "./modules/security"
  vpc_id          = module.networking.vpc_id  # Pass output as input
}

In modules/security/variables.tf:

variable "vpc_id" {
  description = "VPC ID from networking module"
  type        = string
}

This way, the security module receives the value as a variable instead of trying to reference the networking module directly.

After declaring new modules, initialize them:

terraform init

This downloads and prepares all declared modules. If you've recently added module blocks but haven't run init, the modules won't be available.

Use terraform validate to check for syntax errors and scope issues:

terraform validate

This will catch many undeclared module references before you attempt a plan or apply.

If you're working with a single root configuration file that references multiple modules, all module blocks must be in the same file or included via Terraform's configuration merging (same directory).

Example structure:

.
├── main.tf           # Contains module blocks
├── variables.tf      # Input variables
├── outputs.tf        # Outputs
└── modules/
    ├── networking/
    └── security/

All module "..." blocks must be in the root configuration files (main.tf, modules.tf, etc.) at the same level.