First, there are a different possible reasons why you’re having this error (based on riders stories and feedback):
•System detected duplicate accounts
•The card you’re using is already enrolled in another rider account
•System have detected potential “fraudulent” transactions
•If you have a driver account and use the same phone for both UberPartner App and Uber Rider App
•Lastly, you’re using a prepaid card (using prepaid cards for Uber is still hit-and-miss, you can refer to this post on how to use Uber if you don’t have credit card).