The "Advertisers" solution allows all users to connect only one Google Ads account (formerly Google AdWords) to Cobiro. If you have a manager account (MCC) containing multiple Google Ads accounts, you need to select one sub-account to connect to Cobiro.
You can connect an unlimited number of Google Ads accounts to Cobiro but each of your accounts needs to be associated with one separate Cobiro account.
The "Agencies" solution is made for marketing agencies managing numerous Google Ads accounts of their clients. By choosing this solution, our CSS partner Comlyn will create for you a multi-client account (MCA) in its CSS approved Merchant Center. Then you can add your clients' Google Ads accounts and provide them with all of the CSS Partner program benefits.
If you are not the Admin of the Google Ads account that you want to connect to Cobiro, you need the Admin to complete that step. Alternatively, you can ask the admin to give you an admin role. Here is Google Ads Help section About access levels in your Google Ads Account.