Your subdomain i.e. https://mycompany.erpnext.com. Must contain only letters, no spaces, numbers or special characters.
Sad to say, the registration failed and I got a message to contact your email id. Can @admins please help out?
Unfortunately, we have discontinued our free solo account from this month. For testing purpose you can create a one-month trial account from the website.
By the way, from where you got the link for solo account?