If you registered recently, check for your registery comment here: https://toyhou.se/~forums/31551.the-royal-treasury/335158.treasury-account-creation 

Otherwise, check for any old usernames with the list function (tap the #001-100) in the Scale Treasury. Usernames don’t update automatically ^^

I should also mention that you need to manually register (which you can do in the link above), so even if you have received an MYO before, it may not mean you have an account.

You can get in touch with cosmical (who runs the Treasury) here: https://toyhou.se/~forums/31250.contact-us/346969.treasury-questions-and-concerns