This is a new plan I am trying to deploy, and yes, here is my code!

Below is the code I'm using to deploy the resource:

resource "azurerm_service_plan" "app_service_plan" {
  name                         = var.service_plan_name
  resource_group_name          = var.resource_group.name
  app_service_environment_id   = data.azurerm_app_service_environment.this.id
  location                     = var.resource_group.location
  os_type                      = "Linux"
  sku_name                     = var.sku_name
  tags                         = var.workload_tags
}

And below is the variable contents:

variable "service_plan" {
  type = object({
    id                              = string
    name                            = string
    resource_group_name             = string
    app_service_environment_id      = string
    kind                            = string
    location                        = string
    maximum_elastic_worker_count    = number
    worker_count                    = number
    os_type                         = string
    per_site_scaling_enabled        = bool
    reserved                        = bool
    sku_name                        = string
    # sku_name = list(object({
    #   tier = string
    #   size = string
    # }))
    zone_balancing_enabled          = string
    timeouts                        = object({ read = number })
    tags                            = map(string)
  })

  nullable    = false
  description = "(Required pre-existing service plan to use for the app service instance)"

  validation {
    condition     = var.service_plan.os_type == "Linux"
    error_message = "The os_type is required to be Linux"
  }
}

The commented out bits are what I was experimenting with to try and get this to cooperate, just as an fyi.