When installing Windows RDS stand-alone server per user should not be used on a workgroup based server, i.e. there is no domain to authenticate the “per User” users.
There is a warning about it when adding licenses but it does not prevent you from adding them in per User mode.
While it seems odd that this is an issue when there is still Local User authentication available, it is apparently the case and the RDS server will not provide access, or at least will not assign licences. That said, I have not tried it in that mode.
The good thing is that you can change from per User to per Device at any time with the RD Licensing Manager using the Convert Licences option.
Converting licences can be done in either direction at any time so swapping is permitted.