How to fix Error: Error running plan: 1 error occurred in Terraform

The generic error message is just the wrapper. The actual problem details appear right after it. Look for:

1. Line numbers: Check the .tf file and line number mentioned
2. Attribute name: What specific configuration is wrong
3. Error type: Syntax error, invalid reference, missing argument, etc.

Example full error:

Error: Error running plan: 1 error occurred:
* resource.aws_instance.example: invalid or unsupported argument "ami_id"
  on main.tf line 5, in resource "aws_instance" "example":
  5:   ami_id = "ami-0c55b159cbfafe1f0"

The actual error here is "invalid or unsupported argument" - the attribute is called ami, not ami_id.

Run terraform validate to catch syntax and structural errors:

terraform validate

This will show all validation errors without requiring provider credentials or remote state access. It's faster than plan and helps isolate configuration problems:

- If validate passes but plan fails: Check your provider configuration
- If validate fails: Fix the errors it reports before proceeding

Verify you're using the correct:

1. Resource type: Check the Terraform Registry for the exact type name

resource "aws_instance" "example" {
   # Correct resource type
   # NOT: resource "aws_ec2_instance"

2. Attribute names: Use Terraform Registry documentation to verify spelling

ami           = "ami-0c55b159cbfafe1f0"  # Correct
   # NOT: ami_id or image_id (depends on provider)

Visit https://registry.terraform.io to find the exact resource type and argument names for your provider.

Check that all references are valid and declared:

variable "instance_type" {
  type    = string
  default = "t3.micro"
}

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t3.micro"
}

Common mistakes:
- Using old Terraform 0.11 syntax with dollar signs
- Referencing a variable_name that doesn't exist
- Referencing a resource with a name that doesn't exist

Ensure your provider is properly configured:

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }
}

provider "aws" {
  region = "us-east-1"
}

Verify:
1. Provider source and version are correct
2. Authentication is configured (environment variables, credentials file, or IAM role)
3. All required provider arguments are present

Set debug logging for more details:

export TF_LOG=DEBUG
terraform plan
unset TF_LOG

Formatting errors can cause parse failures:

terraform fmt -recursive

This reformats all .tf files in canonical format, fixing:
- Indentation errors
- Missing/extra spaces around operators
- Inconsistent block structure

Then run validate again:

terraform validate