If you have the error text translated, you will see that you are trying to order a product with a combination discount (Kombivorteil) but you do not have an o2 mobile phone contract that entitles you to the combination discount. o2 prepaid products do not entitle you to the combination discount and ordering with a combination discount is therefore not possible. 

Hi @Zulug ,
welcome to our community
I agree with @blablup  that it's probably due to the chosen tariff. You are welcome to order an o2 DSL contract as a new customer. The o2 my Home combo is only for customers with an existing mobile contract. Have you already sent your DSL order in the meantime?
Kind regards
Larissa

Hello!

@blablup thank you for the explanations, it makes sense.

@o2_Larissa there is a discount for the existing customers with which the O2 tariff outstands other providers. I haven’t made a final decision on a tariff and provider yet.

Hi @Zulug,
maybe switching along with your mobile contract would be an alternative.
My tip: sign the DSL contract first and then bring the mobile contract to us or sign a new one, and you'll save 50% on the basic fee for your chosen mobile contract.
Kind Regards,
Larissa

Hello @o2_Larissa,

Thank you for the suggestion! I’ve actually decided to get a mobile contract first and already got a SIM card. I just can’t activate it online for some reason, it pops up an error with something like “Card number isn’t found, please call our support center.” Will call tomorrow, hopefully everything will go smoothly after that. But it’s a different topic of discussion, this we can close now.

Best regards,

Aleksandr