This message occurs because the resource is currently being used on an existing itinerary. You must first remove the resource from any existing reservations, then delete the resource with the below steps:
  1. Navigate to Tickets>Group Sale Setup
  2. Click Resources
  3. Click drop down next to resource and select Delete
  4. Note: If resource is associated to a program, you instead need to click drop down and select Mark inactive

If this is a default resource that is added to each itinerary, you will first need to remove the remove the default resource before deleting or marking inactive.