Azure DevOps Terraform Variable

I have a release pipeline in Azure DevOps with an associated variable group. I am attempting to inject a variable from that variable group into my terraform apply job however, it appears that variable's value is not being substituted as the job is actually trying to inject the name of the variable. See below where I have created the variable:

Here is where I am attempting to inject the variable (this is done via Additional command arguments of the job):

-var "POTENTIUM_ENVIRONMENT=${POTENTIUM_ENVIRONMENT}"

And then below you can see my terraform apply failing:

Just hoping someone can point out a simple reason for why the variable's value is not being utilised here?

Below is an example of a resource that is failing to get created due to the error above:

resource "aws_lb" "internal_load_balancer" {
  name               = "${var.environment}-iALB"
  internal           = true
  load_balancer_type = "application"
  security_groups    = var.ecs_security_group_ILB
  subnets            = var.subnets

  tags = {
    Name        = "${var.environment}-iALB"
    Environment = "${var.environment}"
  }
}

resource "aws_alb_listener" "internal_listener" {
  load_balancer_arn = aws_lb.internal_load_balancer.arn
  port              = "80"
  protocol          = "HTTP"

  default_action {
    type = "fixed-response"

    fixed_response {
      content_type = "text/plain"
      message_body = "The page you are looking for does not exist."
      status_code  = "401"
    }
  }
}