Error: No value for required variable in Terraform Cloud or Terraform Enterprise

Problem

You may encounter the error below during the plan phase of a run in your workspace:

Error: No value for required variable
on XXX.tf line XX:
variable "your_variable_name_here" {
The root module input variable "your_variable_name_here" is not set, and has no default value. Use a -var or -var-file command line argument to provide a value for this variable.

Prerequisites

• You are using Terraform Cloud
• You are using Terraform Enterprise

Causes and Solutions

This error message may be caused by one of these reasons. Please see below each reason for a solution:

• You do not have the value for this variable defined in your configuration code
  • If desired, edit your configuration code so that the variable is defined within it.
• You do not have an entry for this variable within the Variables section of your workspace under the Terraform category
  
  • If desired, add an entry into the Variables section of your workspace
• You have an entry for this variable in the Variables section of your workspace, but the variable is under the Environment category
  • Prepend the name of the variable with TF_VAR_ to force terraform to recognize the variable at the environment level during runs
• You have two entries for this variable within the Variables section of your workspace; one entry under the Environment category, and one under the Terraform category
  • This pertains only to Terraform Enterprise `v202111-1` or newer
  • You are encountering a known issue with the application and the only workaround, until this article is updated, is to rename one of those entries so that their key name is no longer conflicting.
• Your configuration code is within a monorepository, and the workspace this configuration code is associated to does not have the working directory set
  • When viewing your workspace through the UI, go to Settings > General, then provide an appropriate value for the Terraform Working Directory field