Good morning Fabrizio Giorgi,
Microsoft restricts access to the free Microsoft 365 E5 developer sandbox in some cases; not qualifying is often due to account type, prior subscriptions, regional restrictions, or policy limits. To be more specific, I'll explain in details, hope you don't mind reading this long.
1/ Account not a member of the Microsoft 365 Developer Program: You must join the Microsoft 365 Developer Program (directly or via Visual Studio Professional/Enterprise) before requesting a sandbox.
2/ Temporary enrollment restrictions or limited availability: Microsoft sometimes limits or pauses sandbox provisioning (capacity or program policy changes). If the program is restricted at that time, new sandboxes won’t be granted.
3/ Prior or existing sandbox use or conflicting subscriptions: If you already have an active developer sandbox or previously used one recently, you may not be eligible for another until renewal rules or activity checks allow it.
4/ Account type or verification problems: Business/organizational accounts, guest accounts, or accounts without required verification (phone, alternate email) may be rejected. Ensure your account is verified and is eligible for developer program benefits.
5/ Region or tenant policy restrictions: Some features or sandbox provisioning can be limited by geographic region or tenant-level policies set by your organization’s admin.
6/ Improper activity or usage patterns: Sandboxes are intended for development/test activity. Accounts showing only trivial or nondevelopment use may not be prioritized for automatic renewal or new provisioning.
To proceed, you need to: Confirm membership in the Microsoft 365 Developer Program and that your account is fully verified. Retry later—Microsoft occasionally lifts restrictions or increases capacity; the same request may succeed at a later time. If you’re part of an organization tenant, ask your tenant admin to check tenant policies or to create a dev tenant for you. If you believe you meet requirements and still can’t get a sandbox, open a support request or post the specific message you received (copy/paste) on Microsoft Q&A—include your account type, region, and the exact text of the eligibility message for faster help.
I hope you find this comprehensive information useful to some extent. It's really appreciated of you to accept the answer as a way to share your valuable experience with the community. Thank you :)
Vivian