Update: Please follow this guide:
Solution found by @Artem Stivensonn https://customers.lakesidesoftware.com/articles/troubleshooting/Cloud-Add-the-Account-as-an-External-User-in-the-Tenant-First-1632490250805
Hi @Artem Stivensonn , since you're using a custom domain you'll need to invite that email to your tenant. Please follow the linked instructions, and then grant Global Admin permissions so you can manage your tenant.
Please let me know if you have any questions and I can help you further.
If this answer helps you please mark "Accept Answer" so other users can reference it.
Thank you,
James