No suggestions here worked!
What worked was to look for the dump file or execute WatchFaceStudio.exe from command line.
Dump can be found with the help from this post from " [r.liechty_SDP]":
In there you should find where the Studio was looking for the ttf font, just rename and copy your ttf font in there and it should build on your watch, still no correct font in the preview, but it’s OK.
Better this than nothing!