How to fix Error: Postcondition failed in Terraform

The postcondition error includes your custom error_message which describes what went wrong. Read this message to understand what condition failed and what values were expected versus actual.

Example error output:

Error: Resource postcondition failed

  on main.tf line 15, in resource "aws_instance" "example":
   15:   postcondition {

Postcondition failed: instance does not have a public IP address assigned

The error message "instance does not have a public IP address assigned" tells you exactly what the postcondition expected.

Examine the lifecycle block's condition attribute to understand what it's checking. Make sure:
- The condition references the correct resource attributes
- The attribute names match the actual resource schema
- The comparison logic is correct (e.g., != "" vs != null)

Example postcondition that fails:

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

  lifecycle {
    postcondition {
      condition     = self.public_ip != ""
      error_message = "instance does not have a public IP address assigned"
    }
  }
}

If the instance doesn't get a public IP (e.g., no public subnet), this postcondition fails.

Check if the resource configuration actually satisfies the postcondition requirements. For example:
- Is the instance launched in a public subnet? (needed for public IP)
- Do you have the right IAM permissions? (needed for permission checks)
- Is the resource configuration correct? (missing required arguments?)

Run terraform plan to see what will be created:

terraform plan -out=tfplan

Examine the planned resource attributes to see if they'll satisfy your postcondition.

Modify your resource configuration to ensure it will meet the postcondition requirements. Common fixes:

For network-related postconditions:

resource "aws_instance" "example" {
  # ... other config ...
  associate_public_ip_address = true  # Ensures public IP is assigned
  subnet_id = aws_subnet.public.id    # Use public subnet
}

For permission-related postconditions:

# Verify IAM role has required permissions
resource "aws_iam_role_policy" "example" {
  name = "example-policy"
  role = aws_iam_role.example.id

  policy = jsonencode({
    Version = "2012-10-17"
    Statement = [{
      Effect   = "Allow"
      Action   = "ec2:DescribeInstances"
      Resource = "*"
    }]
  })
}

After making changes, run terraform plan again to verify the postcondition will pass.

If the postcondition is checking for something optional or environment-specific, you can:

Remove the postcondition entirely if it's no longer needed:

resource "aws_instance" "example" {
  # ... resource configuration ...
  # Remove the lifecycle block
}

Make the postcondition optional using can() or try():

lifecycle {
  postcondition {
    condition     = try(self.public_ip != "", true)  # Pass if public_ip doesn't exist
    error_message = "instance should have a public IP"
  }
}

Adjust the condition to be less strict:

lifecycle {
  postcondition {
    condition     = can(self.private_ip)  # Just verify private IP exists
    error_message = "instance must have a private IP"
  }
}

After fixing your configuration, run apply to verify the postcondition passes:

terraform apply tfplan

If the postcondition still fails:
1. Check the actual resource attributes that Terraform created (view in AWS console or with terraform state)
2. Compare them against your condition expression
3. Verify the condition logic is correct
4. Update the resource configuration or postcondition and try again

If it succeeds, the resource is created and your infrastructure is valid!