If you already have a Google My Business profile, it will appear as "claimed by others". For Listings to be able to update the profile with the most recent information, you need to be added as an administrator.
If no Google My Business profile exists, one will automatically be created. Then, it is up to you to initiate the verification process. Even if a profile exists but has not been verified by you or your client, the location needs to be verified. In both cases, either a postcard with the verification code will be sent to the registered business address, or a phone call given to the business phone with automatic transmission of the verification PIN. The verification code needs to be entered into the Google My Business account in order to complete the verification process.