This issue happened in the past to several of our customers and took us quite a bit to figure out what is causing it.
We found out that if you're using Custom Forms add-on and have elements within the attendee form that starts with number or some symbol (pretty much anything except alphabetical character) and you have checked Create Ticket Template Element checkbox for some or all of the elements within that custom form, an error will occur and you basically won't be able to access Ticket Templates are of Tickera at all.
However, although we have fixed this in the latest version of Custom Forms add-on, if you have created custom forms using the older version of the add-on the issue will occur.
To resolve this, you can use an add-on provided below which, when installed and activated will add an option to Tickera Settings -> System tab right at the bottom in form of a button labeled Fix Custom Fields. So, all you need to do is to click this button and the issue will be resolved.
This add-on is installed and activated like any other WordPress plugin and once you fix the issue you can safely deactivate and remove it.