Can not see custom content type to add to library

Question

I have Sharepoint 2016 on prem version. I have created a library with Infopath form for employees to complete. Once the form is submitted a workflow needs to assign a task for someone else to Acknowledge receipt. The assign task in the workflow make use of a custom content type. I've created the custom content type with Workflow task as parent.
I need to add that content type to the library in order for the workflow to assign the task. But when I go to Library/settings/ and click on 'add existing content type' it does not show the newly created content type in the list of available content types. Why would that be? (I also noticed some other content types also does not show)

Answer 1

Per my test, the workflow task content type is not supported by a document library, you can only add it to a task list.