@Chen Yuhan
Thank you for the post!
If you aren't receiving the code or notification, you can try going to "aka.ms/mfasetup" and changing your primary means of authentication to phone call or text. This way you can at least authenticate yourself while trying to figure out how to re-add your account to the app.
For the QR code, if your account is not a Microsoft account, I'd recommend looking for a security/2 factor tab on your school account that lets you set up the app with a QR code. For more info.
I've also added the "microsoft-authenticator" tag to this thread so the community here can take a look into your issue as well.
If you have any other questions, please let me know.
Thank you!