Calendar field is not allowing for Action Plan template creation

Summary:

I am trying to create an Action Plan which will be produced in a case. However I can not set the calendar field when I go into Setup and Maintenance > Service > Manage Action Plan templates > Create Template

I have also set up the Schedule calendar under the Manage Availability within Subscription Configuration and it is still not allowing me to create a template

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):