@TobiasL6952 Thank you for reaching out to us, As I understand you are trying to access newly created Defender EASM instance and getting 403 error.
Would request you to check if Owner or Contributor RBAC role either on the subscription or resource group level?
You don't need Azure AD roles (i.e. GA) to create, delete, and edit MDEASM resources and the inventory assets within it. They need RBAC role (IAM) - https://learn.microsoft.com/en-us/azure/external-attack-surface-management/#:~:text=new%20cloud%20resources.-,User%20permissions,-Users%20that%20have
Let me know if you have any further questions, feel free to post back.