Error: Module not installed

Fix terraform "Error: Module not installed"

TL;DR: You added a module block, or changed a module's source, after the last terraform init. Module installation happens only during init; plan never installs. Run terraform init again, and re-run it every time you touch a module block.

The error

Error: Module not installed

  on main.tf line 11, in module "ada":
  11: module "ada" {

This module is not yet installed. Run "terraform init" to install all modules required by this configuration.

Steps

1. Run terraform init. Expected: output shows the new module being installed into .terraform/modules, ending with Terraform has been successfully initialized!
2. If you changed a source (new registry version, new git ref), run terraform init -upgrade so the cached copy is refreshed. Expected: the module is re-downloaded at the new source.
3. Verify the install manifest: check .terraform/modules/modules.json lists your module. Expected: it is there.
4. Run terraform plan. Expected: the plan proceeds instead of erroring.

When this applies

• plan or apply fails with "Module not installed" right after you added a module block, changed its source, or cloned the repo fresh.
• CI pipelines that run plan without an init stage fail the first time anyone adds a module.

When it does NOT apply

• Registry resolution errors for a module source ("module not found"). That is a source-address problem, not a missed init.
• Backend initialization errors. Those are about state storage, not module code.

Tool and version compatibility

• All Terraform CLI versions. The init-installs-modules rule predates 0.12 and is unchanged.

Why it happens

init copies or links each child module into .terraform/modules and records it in the manifest. plan and apply only read that manifest; they never fetch. A module block added after init is simply unknown to the working directory until init runs again.

Edge cases and pitfalls

• Changing only the module's version or git ref still requires re-init; the cached copy is pinned to the old one.
• Every CI pipeline needs a dedicated terraform init -input=false stage before plan. It is cheap and idempotent, and it also downloads any new providers the module pulls in.