You can’t build without having an Author and Distributor Certificate. So error 12 is one of two things
You updated operating system and your certificate needs to be recreated or in this case you need to accept the RSA Encryption Key in order to decode the certificate.
Restart your watch by holding down the power off button for about 15 seconds until it says restarting.
Restart your computer to clear any caches (don’t just turn off an on).
make sure your watch has debugging and developer options enabled
set your watch face to a factory installed watch face (this helps keep the RSA Acceptance show longer)
Not run on device from GWS
Keep your watch active and watch it like a hawk
A dialog will come up to accept the RSA Encryption key it will have don’t show again but it won’t show again no matter if that is checked or not. OK it
The watch face should then transfer to your watch.
Ron
Samsung Developer Relations