A cloud-based identity and access management service for securing user authentication and resource access
Hey there, Momna khan
When you say you get an AADSTS16000 error when signing in with your university account, this suggests the account you're using is not recognised in the tenant you're attempting to access.
When your university account was a guest user, authentication was handled through your university's Entra ID as a federated sign-in method. Your account would also have had a User Principal Name (UPN) which uses your university email address with the external (#EXT#) value followed by your tenant name on an .onmicrosoft.com domain.
Once you convert your account from an external account to an internal member, the federated sign-in method is removed, leaving just the UPN as the sign-in method for that account. As a result, signing in with your university account may now be directing you to your university's tenant rather than the Azure for Students tenant that contains your subscription.
The good news is the subscription will still be tied to the account and tenant. However, you will need to sign into your account with the account's UPN rather then the email address and password for your university account.
Please can you try signing in using the UPN of the member account in your Azure for Students tenant and see if you can access the Microsoft Entra ID blade here: https://portal.azure.com/#blade/Microsoft_AAD_IAM/ActiveDirectoryMenuBlade
Hope this helps,
Nathan