Problem:
Saving custom post type throws Error:
You cannot use this slug because it is already used or a reserved word. Please choose a different slug.
Solution:
As the error reads, you must have created either any post, page, content template or view any any other entity with the slug "tablet-application" or it's a reserved slug.
WordPress do not allow duplicate slugs and slugs should be unique. Can you please try to find where you are using the slug tablet-application" and then rename that slug and then it will allow you to create entity with your desired slug.
