In order to create an Assignment in Prep, you must first make sure you are assigned the appropriate user-role access. Only users who are assigned as either a Professor, Location Admin, or Account Admin will have the ability to create Assignment interviews.
If you have created your user access by registering your own account on the login landing page, please note, that your user access will be automatically assigned as "Student".
If you are meant to be a Professor, Local Admin, or Account Admin, you should contact your internal team lead, another Account Admin, or your designated Client Success Manager. Only these individuals will be able to assign you a different user role other than "Student".