Can't login using username created for new spotify account

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Hey guys. I wanna know why I can't login using the username I created for my new account? I have tried on the app and the website but I always login using my email and it kinda makes me sad cos who wants to type their email all over again just to login to spotify. I can't even change my username from your system generated alphanumeric username showing on the website. I hope you could help me. Thank you!!!

P.S. I'll keep my chosen username first until I get answers. Thanks!