You need to be using your educational organization's school email address for the free student account:
From the FAQ:
What do I need to sign up?
You need to provide a phone number and your academic organization’s email address (for example, your_name@your_school.edu) to sign up for an Azure for Students subscription.
If you are already using that and still facing this issue, try using a new browser in incognito mode, or removing Azure URLs from your cache. It's possible that an old login setting might be getting stored.
The tenant ID and subscription ID are different things, but if the tenant ID in the error message is different from the tenant ID where you set up the subscription, then you might be logging into the wrong tenant due to some saved browser settings. You can switch to the proper tenant from the top right corner of the portal.