It seems the link of changing account type(Sandbox) is broken and there is no one for support or even any response to this problem. no help, no support, no response and even no way to contact.
Please file a contact us support request here https://developer.ebay.com/contact-us/#help-with-the-ebay-sandbox under section "Help with the eBay Sandbox" and a concern team member will do it for you and will inform you via your requested email address.
Best Regards ,
eBay Developer Support
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