When I try to rally to fight Wendell the clown it says “ no idle units” what do I do?

No idle units is a message you get when trying to use the 'Automatic Deployment' option. This button will try to assign troops to your APC's depending on what you have currently assigned. It only works though if you have some spare unassigned troops to use, if not you get the 'no idle units' message.

To rally the clown you need to tap one to select it. Choose the time and then pick an available APC and click March. Your alliance members will then need to join you before the time runs out. There are some requirements though, more details here:

https://www.appgamer.com/last-shelter-survival/strategy-guide/how-to-rally-against-wendell-the-clown