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Your Gmail account is still associated with your employer if you are seeing that sign in prompt.
You need to cancel that popup and switch directory/tenant if you can, or sign in to the Engage Hub with a different email address
https://aka.ms/credentials-support takes you to the Engage Center Hub, the location for certification support. I sometimes find that I need to use https://aka.ms/credentials-support twice. If you do not see Microsoft Credentials selected against Product, create an account on the Engage Center Hub and then use the direct link https://aka.ms/credentials-support again. If you see the message "You are currently logged in as a guested personal account, which may result in access issues on Engage Center." then click on "Sign in to Microsoft's tenant for Personal Users", select "Login to Engage Center", and then use the direct link https://aka.ms/credentials-support again. If "No Access" is displayed, select Switch Tenant and choose your own tenant.