Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Try an in private browser window (press Ctrl+Shift+N in Google Chrome or Microsoft Edge) and see if you are able to sign in with the correct account.
Typically, during the initial setup of a business subscription, you create .onmicrosoft.com user accounts and one of them is the admin who should sign in via https://admin.microsoft.com .
Alternatively/also, try Microsoft business phone support. Go to https://support.microsoft.com/en-us/topic/customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2 .