I faced this issue before. I created a template using Task list and it was not appearing in add an app.
After some search on google I found that some of list templates hidden by design and task list was one of them.
The explanation for the template not being displayed can be one of the following ( except invalid templates of course, but then you would get a full-blown error ) :
- the language of the template is different than the language of the site where you want the template to be used.
- the version of the template does not match the minimum version difference accepted by the site where you are attempting to use it ( ex. you are saving a SharePoint 2007 list and are trying to use it in a SharePoint 2013 environment)
- the template is hidden by design
Below are the list templates are hidden by design:
- Project Tasks
- Data Sources
- Converted Forms
- No Code Workflows
- Status List
- Workflow History
- No Code Public Workflows
- Persistent Storage List for MySite Published Feed
- Custom Workflow Process
For reference: List saved as template does not appear under Add an app