@Sirée Koolen-Wijkstra , As I understand you're not using your @ outlook account.
You need to sign in with your Microsoft account, such as an Office 365 or Outlook.com account for sandbox access.
Please check this FAQ: How do I activate the sandbox?
The first time you activate a sandbox and accept the terms, your Microsoft account is associated with a new Azure directory named Microsoft Learn Sandbox, and you're added to a special subscription named Concierge Subscription. Each time you activate a sandbox, a single resource group is created for you. You can use it to create and manage all the resources (for example, virtual machines, storage accounts, and app services) that you need for a module.