Delete a "Custom application" in "Application Manager" at "Systems Manager"

For anyone else like me that was still confused by what "You can't delete an application in Application Manager, but you can remove an application from the service by deleting the underlying tags, Resource Groups, or AWS CloudFormation stacks." means, here's what I was doing and what I eventually figured out. Details below, but basically, I just had to delete Resource Groups that I didn't know existed.

Like OP, I was just testing Application Manager out to see how it works and created a custom application for resources with tag of "Name" with value of "auth". To me, the wording "deleting the underlying tags, Resource Groups" meant deleting the Name tag from the actual EC2 instance, which didn't make sense because I need that tag on the instance. And I didn't think about Resource Groups because I hadn't created any, or so I thought. Turns out creating a custom application from Application Manager automatically creates Resource Groups in the background. Once I deleted the automatically created Resource Groups, the custom application was gone.

This documentation is clear about that. I didn't see it. This Application doesn't have associated resources. And its name is "test." LOL. Thanks for your help.

You might have an item in "Resource Group" that's showing up as an application. Clear that out and it should go away.