If you don't have a credit/debit card the easiest solution would be to create a free debit card which is linked to your PayPal account here: https://www.paypal.com/us/digital-wallet/manage-money/paypal-debit-card
The card will be automatically linked to your PayPal budget, and if you have your bank account linked to your PayPal it will also be linked to your bank account. So when you purchase the Deutschlandticket it will either charge your PayPal balance or your linked bank account.
To see your PayPal debit card details simply open your PayPal wallet and click on your PayPal balance. You should be able to see your card there where you have multiple option. You can click on the Eye symbol on your card to see your card details which you can use to pay for your Deutschlandticket.