You cannot log in to Young Platform with the same credentials you use for Step.
They are two different platforms, so you will have to create a separate account for Young Platform.
We recommend using the same email address you used to sign up with Step, but it's not compulsory.
You will be able to transfer YNG tokens from Step to Young Platform even if your registration email is different.
Find out how to create a Young Platform Account.
Find out how to transfer YNG Tokens from Step to Young Platform.