There are chances where while creating a technician it might not show any account or any site. If that is the case then we won't allow to save that technician page without associating any account and site, if login is enabled for that technician.
If all the sites in all the accounts are Referring to default settings, this happens i.e. Under Admin>Sites>Edit the Site>Check Refer Settings.
Let me explain the reason behind this.
Say for example in a particular account there are 5 sites and all the sites are referring default settings. It means that all the 5 sites are referring default technicians as well. So there is no point in showing this site while creating a technician as this technician may be a default tech or may not be. Hence this limitation. Also if all the sites are in refer we won't show all the sites so again there is no point in showing the account of those sites.
It can also be solved by creating a dummy site for the account and set the related settings to "Copy Default Settings" or "Custom Settings" to list the account and sites to associate to a technician or simply you can change anyone site in any one account to Custom or Copy default which also helps in this case.