It is most likely an account issue. Make sure you are logged in with the correct Microsoft account that has access to the sandbox. Try logging using an inPrivate Browser.
See how to activate sandbox here: How do I activate the sandbox?
https://learn.microsoft.com/en-us/training/support/use-your-own-subscription
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola