I’m also facing the same issue.
Modern robots means they run in the context of a certain user identity. I have created one machine and one attended user linked with same machine. Then i just paired the machine key with UIPath Assistant and robot automatically adds up there.
Yes i think so that in community version we cant add robots manually because i am still unable to add robots.
Try creating one machine and pair it with UIPath assistant. Also add one user and modify its roles and folder permissions. Above screenshots can help you.