Hi Constanza,
I would implement it over the current user. If a user can only have one ticket in total, you can check over XPath if there is no association between the user entity and the ticket entity.
[not(MyFirstModule.Ticket_User/MyFirstModule.User)]
If a user can have more tickets I would implement the Ticker to have the Store createdDate in the Entity implemented and in the microflow check if the user didn't create a ticket in the last <x> minutes.
Go Make It