United Airlines will not allow you to cancel a basic economy ticket unless it is within 24 hours from the time it was purchased, and was purchased a week or more before departure. If it has been more than 24 hours since you booked your ticket, you can still cancel your ticket and get a full refund, but it will come in the form of a flight credit toward future travel with United
Source: https://www.devex.com/people/what-is-the-cancellation-policy-for-united-airlines-1954288