If your AQI (Air Quality Index) complication is only showing ‘–‘ instead of the real a value, then it’s probably because Apple does not have the AQI data for your selected city. See below the difference between a city that does have AQI data (Gold City) and another one that doesn’t (Brussels) – you can check it for yourself in the default iOS Weather app:

A few tips:
1. to change the default city on the Apple Watch, open the Watch app on your iPhone and go to My Watch / Weather / Default City.
2. if the default iOS Weather app does not provide AQI data for your city, there is still a chance that other apps – like Air Matters – will do. If that’s the case, you will need to install that app on your watch and use its complication to show the AQI on your watch face
3. if you are interested in the quality of your indoor air, get an air purifier and air quality monitor, like the AirVisual Pro.