Hi Michal,
the domain added in the verification process needs to be in your shopware account aswell, running localhost is no problem (i am using projectdir.test for example) but you will need to have a verified domain in your shopware account beforehand.
If you have struggles setting up the domain in your account, just contact my colleagues from the sales department: https://www.shopware.com/en/contact/contact-sales/
Just for your information: each domain in our system is unique, that is why you would not be able to use „localhost“ as a domain.
Kind regards,
Tim