Outlook Exchange and Business Basic

The reason you can't sign up for Microsoft 365 Business Basic with your current email is because that email is already part of an existing Microsoft 365 tenant managed by your vendor. Microsoft doesn't allow the same email or domain to be used across multiple tenants. Since your vendor controls the account tied to Outlook/Exchange, you're blocked from purchasing Business Basic directly from Microsoft using that email. To resolve this, you can either ask your vendor to assign a Business Basic license (if they support it) or create a new Microsoft 365 account using a different email or domain.