How to fix Invalid version constraint - This string does not use correct version constraint syntax in Terraform

The error message should indicate which version constraint is malformed. Look for:

- required_version in the terraform block
- version in the required_providers block
- version in module blocks

Example of a properly structured terraform block:

terraform {
  required_version = ">= 1.0"
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }
}

Check your configuration files (.tf files) for the referenced field and note the exact constraint value.

Common mistakes and their fixes:

Wrong - Empty string:

required_version = ""

Correct:

required_version = ">= 1.0"

Wrong - Undefined value:

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = undefined
    }
  }
}

Correct:

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

Terraform supports these version constraint operators:

| Operator | Meaning | Example |
|----------|---------|---------|
| = or none | Exact version | = 1.5.0 or 1.5.0 |
| != | Not equal | != 1.4.0 |
| > | Greater than | > 1.0 |
| >= | Greater than or equal | >= 1.5.0 |
| < | Less than | < 2.0 |
| <= | Less than or equal | <= 1.9.0 |
| ~> | Pessimistic (rightmost increments only) | ~> 1.5 allows 1.5.0-1.9.9 but not 2.0 |

Examples of valid constraints:

terraform {
  required_version = ">= 1.0, < 2.0"
}

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.15"     # Allows 5.15.0 to 5.99.x
    }
    google = {
      source  = "hashicorp/google"
      version = ">= 5.0, < 6.0"
    }
  }
}

Version numbers must follow semantic versioning: MAJOR.MINOR.PATCH

Valid version numbers:
- 1.0.0
- 1.5 (patch defaults to .0)
- 1 (minor and patch default to .0)

Invalid version numbers:
- 1.0.0.0 (too many components)
- 1.x.0 (x is not valid, use comparison operators instead)
- latest (not supported, use constraint operators)
- v1.5.0 (remove the v prefix)

Correct ways to specify versions:

# Exact version
version = "= 1.5.0"

# Minimum version (common for modules)
version = ">= 1.0.0"

# Version range
version = ">= 1.5.0, < 2.0.0"

# Pessimistic constraint (recommended for providers)
version = "~> 1.5"

After fixing your version constraints, validate your configuration:

terraform validate

If validation passes, you can proceed to initialize:

terraform init

If terraform validate still fails, check the error message again—it will pinpoint which constraint is still invalid. Common issues to double-check:

- No leading/trailing spaces in version strings
- All quotes are properly closed
- Operators are spelled correctly (~> not ~>> or ~, >=, not >==)
- Version numbers use dots, not commas between components (1.5.0 not 1,5,0)

When using modules, you can specify version constraints. Ensure the syntax is correct:

Wrong:

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = ""  # Empty constraint
}

Correct:

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "~> 5.0"  # Allows 5.x.x
}

For local modules (relative paths), you typically omit the version constraint:

module "custom" {
  source = "./modules/custom"
  # No version constraint for local modules
}

Registry modules (terraform-aws-modules, etc.) should have version constraints to ensure stability.

If you see the error alongside an invalid source URL, ensure the provider source is correct:

Wrong:

terraform {
  required_providers {
    aws = {
      source  = ""  # Empty source
      version = "~> 5.0"
    }
  }
}

Correct:

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"  # namespace/type format
      version = "~> 5.0"
    }
  }
}

Use the [Terraform Registry](https://registry.terraform.io) to find correct provider sources and compatible versions.