Usually, all the orgs use multiple PTO types for users, like Vacation, Sick, Personal, etc., 

If there are multiple PTO Types set up for you, then you can select the PTO type and request for the PTO.

If there is only one PTO type setup for you, then you should scroll the PTO list even though you see only one PTO type and then select the PTO type.

We suggest you scroll and then select the PTO type is because if you select the PTO type without scrolling the list, then the PTO type is not selected and you might see a message as "Please select PTO type" and not able to request a PTO on the Time Tracker for Salesforce Mobile app.