Hello BenettBolek,
When a company or school signs up for an Office 365 for Business subscription, Microsoft will create a new domain with this onmicrosoft.com domain for the company. For example, @contoso.onmicrosoft.com. The Office 365 admins of a company can create new user accounts with this @contoso.onmicrosoft.com domain and manage the data under this tenant.
May I know what application you activated and what error message you see? Please capture some screenshots for further check so that I can understand your scenario.
Regards,
Rick