Based on this old thread :
As mentioned here and in this post as well below might be the reason that you are getting this error:
The error is due to an unmanaged AAD tenant issue and offer the following steps to sign up:
Open an InPrivate session of Internet Explorer and go to (https://imagine.microsoft.com/azure). Log in using your Microsoft Personal Account (if you don't have one, create it from http://signup.live.com). Complete the verification process using your Microsoft School Account address or activation code. Continue the activation of the Azure plan using the Microsoft Personal Account.
Also there is a possibility Your educational institute is not recognized/partnered or eligible with/for Microsoft. You can refer to the FAQ page OR your school admin might have not granted access to you to register for Azure for Students.