Set the key_id attribute in aws_api_gateway_usage_plan_key resource to a valid key id.

Example Configuration

resource "aws_api_gateway_rest_api" "api733" {
  body = jsonencode({
    openapi = "3.0.1"
    info = {
      title   = "example"
      version = "1.0"
    }
    paths = {
      "/path1" = {
        get = {
          x-amazon-apigateway-integration = {
            httpMethod           = "GET"
            payloadFormatVersion = "1.0"
            type                 = "HTTP_PROXY"
            uri                  = "https://ip-ranges.amazonaws.com/ip-ranges.json"
          }
        }
      }
    }
  })
  name = "API733"
}

resource "aws_api_gateway_api_key" "api_key_733" {
  name    = "APIKey733"
  enabled = true
}

resource "aws_api_gateway_deployment" "dep_733" {
  rest_api_id = aws_api_gateway_rest_api.api733.id
}

resource "aws_api_gateway_stage" "stage733" {
  deployment_id = aws_api_gateway_deployment.dep_733.id
  rest_api_id   = aws_api_gateway_rest_api.api733.id
  stage_name    = "stage733"
}

resource "aws_api_gateway_usage_plan" "usage_plan_733" {
  name = "UsagePlan733"

  api_stages {
    api_id = aws_api_gateway_rest_api.api733.id
    stage  = aws_api_gateway_stage.stage733.stage_name
  }
}

resource "aws_api_gateway_usage_plan_key" "usage_plan_key_733" {
  key_id        = aws_api_gateway_api_key.api_key_733.id
  key_type      = "API_KEY"
  usage_plan_id = aws_api_gateway_usage_plan.usage_plan_733.id
}