Hi Edoardo,
Thanks for using Microsoft products and posting in the community, I'm glad to offer help.
Theoretically, if we want others to see the embedded content, the resource must be public. In your case, since the workbook is stored in your personal OneDrive, it is inaccessible to anyone else.
To solve the problem, you can try to share the workbook to make it become a public file and then embed it on your website.
Reference: Share OneDrive files and folders - Office Support
However, as I don’t know how you embedded the workbook, I’m not sure if the above method would work, but I hope it can give you some guidance.
Please feel free to let me know if there is any progress.
Best Regards,
Arck