How to fix Error: Precondition failed in Terraform

When a precondition fails, Terraform displays the error_message you defined in the precondition block. This message should explain why the condition failed.

Error: Resource precondition failed

  on main.tf line 42, in resource "aws_instance" "example":
   42:   precondition {

condition = contains(["us-east-1", "us-west-2"], var.aws_region)
var.aws_region is "eu-west-1"

Your message: AWS instance can only be created in us-east-1 or us-west-2

The error shows:
- Which file and line the precondition is on
- The condition expression that failed
- The actual values involved
- Your custom error_message

Study this output to understand what assumption was violated.

Review the variables referenced in your precondition. Use terraform console or print statements to check their actual values:

terraform console
> var.aws_region
"eu-west-1"
> var.environment
"production"

Or check your terraform.tfvars or environment variables:

# Check what variables are being passed
cat terraform.tfvars

# Check environment variables
env | grep TF_VAR

Ensure the values passed match what your precondition expects. If using automation, verify that the correct variables are being provided.

Check your precondition block in the resource definition. Verify the condition expression is correct:

resource "aws_instance" "example" {
  # ... configuration ...

  lifecycle {
    precondition {
      condition     = can(data.aws_ami.ubuntu.id)  # Ensure data source exists
      error_message = "Ubuntu AMI data source lookup failed."
    }
  }
}

Common mistakes:
- Using wrong operators (== instead of contains, != instead of !=)
- Referencing properties that don't exist on the object
- Using self (not allowed in preconditions—use resource references instead)
- Logic errors in boolean expressions

Test the condition expression independently using 'terraform console' if needed:

terraform console
> contains(["us-east-1", "us-west-2"], var.aws_region)
true  # or false, depending on the variable

Once you understand why the precondition failed, fix it by either:

Option 1: Update the input variables
Update your terraform.tfvars or pass different values:

# terraform.tfvars
aws_region = "us-east-1"  # Change from eu-west-1
environment = "production"

Or via CLI:

terraform apply -var="aws_region=us-east-1"

Option 2: Modify the precondition logic
If the condition is too restrictive, update it in your configuration:

lifecycle {
  precondition {
    # Add eu-west-1 to allowed regions
    condition     = contains(["us-east-1", "us-west-2", "eu-west-1"], var.aws_region)
    error_message = "AWS instance can only be created in specified regions."
  }
}

Option 3: Check data source prerequisites
If the precondition checks a data source (like looking up an AMI), ensure that data source can resolve:

terraform plan  # Verify data sources fetch successfully
terraform console
> data.aws_ami.ubuntu.id  # Check if it resolves

Once you've adjusted your configuration:

# Generate a new plan to verify the precondition now passes
terraform plan

# If the plan looks good, apply the changes
terraform apply

Terraform will evaluate the precondition again. If it passes, the plan will proceed to creating/modifying resources. If it still fails, you'll get the same error and can iterate further.