PBS does not offer free trials, however you may have obtained the Amazon Prime trial. The PBS Passport membership is a separate service from Amazon Prime.
You may notice both PBS and Amazon offer some (but not all) of the same programs for online streaming. This is because PBS and Amazon are both distributors of content and must purchase streaming rights for certain programs, and sometimes both PBS and Amazon purchase streaming rights for the same program. PBS and Amazon are not connected even though some of the same programs are offered by both.
If you have purchased a subscription to Amazon Prime or trial, this can only be used at Amazon's site and apps. You will have made an account specifically for Amazon. Both this account and service cannot be used at PBS. Likewise, a PBS account and Passport can not be used at Amazon's site or apps.
If you are trying to use Passport, you must sign in at http://www.pbs.org or a PBS Video app.
If you have questions about Amazon Prime or using Amazon Prime, please contact Amazon support: https://www.amazon.com/support