are you trying with an user account that has some managerial role like editor?
I mean the current code should check if self-registration is available when it creates a list of available author roles to the submission form: pkp-lib/PKPSubmissionSubmitStep1Form.inc.php at master · ajnyga/pkp-lib · GitHub
Do you see a list of avalaible roles in the submission form step 1?
But I actually think that the key here would be an option to just turn off submissions in a journal like suggested here: How do I disable submission option?