Terraform Apply Error: Code="InvalidResourceName" Message="Resource name is invalid" When creating VPN-GW Module

Question

I'm receiving the following error when executing the Terraform Apply Command.

│ Error: Creating/Updating Virtual Network Gateway: (Name "VPN-GW-HUB-dev" / Resource Group "RG-HUB-dev"): network.VirtualNetworkGatewaysClient#CreateOrUpdate: Failure sending request: StatusCode=400 -- Original Error: Code="InvalidResourceName" Message="Resource name  is invalid. The name can be up to 80 characters long. It must begin with a word character, and it must end with a word character or with '_'. The name may contain word characters or '.', '-', '_'." Details=[]
│ 
│   with module.MOD-VPN-GW.azurerm_virtual_network_gateway.vpngw,
│   on Modules/10.VPN-GW/main.tf line 31, in resource "azurerm_virtual_network_gateway" "vpngw":
│   31: resource "azurerm_virtual_network_gateway" "vpngw" {

What was I trying to do?

My Terraform code creates a complete Hub-and-Spoke infrastructure in Azure. It worked very well until I decided to modularise the VPN GW part.

My Terraform code structure is the usual format:

--Modules/{module-name}/main.tf
--main.tf

So within the {module-name} - I have a folder called:

10.VPN-GW\main.tf

What have I tried?

Well, I have tried to reformat the code, I double checked it against all other modules that worked OK. I have NO idea why this error is happening, and it has been 3 days now non stop debugging this.

The module file is here:

#------------------------------
# VPN GW MODULE FILE - ALso contains GW SUBNET
#-------------------------------

resource "azurerm_subnet" "gwSubnet" {
  name                 =    var.subnet_name
  resource_group_name  =    var.subnet_resource_group_name
  virtual_network_name =    var.subnet_virtual_network_name
  address_prefixes     =    var.subnet_address_prefixes

}

variable "subnet_name" {
    type = string
}

variable "subnet_resource_group_name" {
    type = string
}

variable "subnet_virtual_network_name" {
    type = string
}

variable "subnet_address_prefixes" {
    type = list(string)
}


output "outSubnetIDVPNGW" {
    value = azurerm_subnet.gwSubnet.id
}
 

resource "azurerm_virtual_network_gateway" "vpngw" {
  name                = "${var.vpn-gw-name}"
  location            = "${var.vpn-gw-location}"
  resource_group_name = "${var.vpngw_resource_group_name}"

  type     = "Vpn"
  vpn_type = "RouteBased"
  
  active_active = false
  enable_bgp    = false
  sku           = "VpnGw1"

  ip_configuration {
    name                          = "${var.ip_configuration_name}"
    public_ip_address_id          = "${var.ip_configuration_public_ip_address_id}"
    private_ip_address_allocation = "Dynamic"
    subnet_id                     = azurerm_subnet.gwSubnet.id
  }

    vpn_client_configuration {
        address_space = "${var.vpn_client_configuration_address_space}"

        root_certificate {
        name = "${var.root_certificate_name}"
        public_cert_data = "${var.cert}"
    }

    revoked_certificate {
      name       = "${var.revoked_certificate_name}"
      thumbprint = "${var.revoked_certificate_thumbprint}"
    }
  }
}

variable "vpn-gw-name" {
    type = string
    //default = ""
}

variable "vpn-gw-location" {
    //default = ""
    type = string
}

variable "vpngw_resource_group_name" {
    default = ""
}

variable "ip_configuration_name" {
    default = ""
}

variable "ip_configuration_public_ip_address_id" {
    default = ""
}

/*
variable "ip_configuration_private_ip_address_allocation" {
    //default = ""
   // type = string
}
*/

variable "vpn_client_configuration_address_space" {
    default = []
}

variable "root_certificate_name" {
    default = ""
}

variable "cert" {  
    default = ""
}

variable "revoked_certificate_name" {
  default = ""
}

variable "revoked_certificate_thumbprint" {
    default = ""
}

variable "ip_configuration" {
    type = map(string)
}


variable "vpn_client_configuration" {
  type = object({
    address_space           = list(string)
    root_certificate        = map(string)
    revoked_certificate     = map(string)
  })
} 

/*
variable "revoked_certificate" {
  type = map(string)
}
*/

The main.tf file -- that calls each module, is given below - (I'm only adding the snippet for the part causing the error) - i.e. creation of the VPN GW

#-----------------------------------------
# Create GW SUBNET & VPN GW
#-----------------------------------------

module "MOD-VPN-GW" {
  source              = "./Modules/10.VPN-GW"

  subnet_name                 = "GatewaySubnet"
  subnet_resource_group_name  = "RG-HUB-${var.environmentCode}"
  subnet_virtual_network_name =  "VNET-HUB-${var.environmentCode}"
  subnet_address_prefixes     = ["10.0.1.0/27"]

  vpn-gw-name                 = "VPN-GW-HUB-${var.environmentCode}"
  vpn-gw-location               = "westeurope"
  vpngw_resource_group_name   = "RG-HUB-${var.environmentCode}"
  
  ip_configuration = {
    name        = "VNetGatewayConfig"
    public_ip_address_id                    = "${module.MOD-VPNGW-PIP.id}"
    private_ip_address_allocation  = "Dynamic"
    subnet_id                   = module.MOD-VPN-GW.outSubnetIDVPNGW
  }

  vpn_client_configuration = {
    address_space = ["172.16.10.0/24"]

    root_certificate = {
      name = "dev.vpn.macos.com"
      public_cert_data  = data.azurerm_key_vault_certificate.akv-certificate.certificate_data_base64

    }

    revoked_certificate  = {
      name       = "Verizon-Global-Root-CA"
      thumbprint = "912198EEF23DCAC40939312FEE97DD560BAE49B1"
    }
  }

   depends_on = [
     module.MOD-RG-HUB, module.MOD-VNET-HUB, azurerm_linux_virtual_machine.label-vm-spoke-01
  ]

}

What did I search for?

I cannot seem to find this particular error code in a google search. Its not telling me that name given to my VPN GW is incorrect - its giving me the name (in the error message) and telling me that there is no name given. Which is odd. Or have I misunderstood the error message?

The terraform init command succeeds. The terraform validate command succeeds. The terraform plan command succeeds.

This fails when applying