Hi Matthew Stewart,
Thank you for reaching out to us on the Microsoft Q&A forum. From your description, it seems your issue is with Microsoft 365 E5 Sandbox.
You can refer the following link: Do not qualify for developer sandbox? Needed for Learn Modules - Microsoft Q&A.
For more updates follow our blog: Microsoft 365 Developer Program.
Note: Microsoft 365 Business Basic Trial offers a complimentary one-month trial period when you provide your credit card details for future billing.
Hope the above information can help.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Thank you