Hi Enzo1919,
Welcome to the PSR Tutorial Forum!
To create your own Voices, you must first create a new, empty Pack (My Packs > Create Pack). If this Pack is selected under "My Packs", click on the lower "+" sign to create a new, empty Voice in this Pack (Create Normal Voice or Create Drum Voice). Double-click the new Voice to open the Voice Editor.
Note: Only Preset Voices of Genos and Tyros5 can be edited in YEM. Unfortunately, this is not possible for PSR-S and SX models. On the SX900 you only have the opportunity to create your own Normal and Drum Voices that use your their own wave samples.
P.S.
Of course, you can also use an existing (and not protected) Pack and create new Voices there. It is important that you then save your finished Voices as a .ppf Pack file (either the complete Pack via "My Packs > Export Pack" or individual Voices of a Pack via "> Export Selected Content as Pack").
Best regards,
Chris