Is the computer indeed password-less? Windows 10 usually requires you to first set a password before you can add any other authentication options such as PIN or fingerprint or any other biometrics. PIN is allowed to be used only via local interactive login (or elevated UAC prompt), biometrics should be similar, so a fallback to a password may be necessary not just in case of remote logging in (such as via RDP) but also in case of a significant hardware change (such as replacing a MOBO or moving the entire HDD/SSD to a new computer as-is without reinstall).
Any remote login when a real password-less account is created (such as when installing new Windows and not entering any password during the OOBE) is blocked by Windows, so if it happens that an account without any password exists (although by some means, PIN or biometrics is used), Windows would probably still block any external login. Since SSH server is quite a new feature on Windows, brought from the Unix world, it may not be prepared for the situation of a passwordless account but with a SSH key.
The workaround then should be to create a password (even if a 32char random one) thus removing the block for remote login.