Have you set up a Mein o2 account with the prepaid cellphone number?
If you want to redeem a top-up code, you can also do this with *103*top-up code# (press the green dialing button on your phone afterwards)

I have been trying through website and not the recharge card or top up card. I just wanted to top up with 10€ but O2 I couldn't find card with 10€ card. 

Top-up codes can be bought from an amount of 15 euros.

You can use “bank transfer”

Send a blank SMS to 56656 with you phone and you will get a SMS with the IBAN of Telefonica/o2 and your personal intended use. You can transfer any amount.

You can enter your bank details in Mein o2 for automaic top-up

https://www.o2online.de/ecare-prepaid/automatische-aufladung

The credit is automatically topped up when the credit is less than 4 euros and/or

The balance is topped up on a specific day of the month and/or

The money for an option will be charged from your bank account if your current balance is not sufficient for activation.

Hello @Boww's , welcome in our community. 
Were you able to top up your account with the advises from tearsbeforecrying?
Thanks for your support tearsbeforecrying.

kindly regards
Michael