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Do you have license file for Mathcad Prime 6.0? If yes, try to configure the same with Mathcad 15.0, it should work.
And, if you do not have license file already then contact PTC Web Account Administration team by logging a case here https://support.ptc.com/support/feedback/web-account.htm and request to reset your password.
Once, password is reset, check if it helps.