I got my Nova Air some days ago and was confronted to the same problem. I asked to Boox help and they provided me a document to successfully activate Google Play Store. Basically you have to go to settings, application info, then use the upper 3 dots to activate a menu called "show system". Then you have to "clear storage" in the "storage and cache" section for 1) google play services 2) google play store and 3) google services framework.Then you can activate Google Play in the main settings, login to your account, register the GSF ID and it worked for me.
The point is that for some reason, moon reader is not available in Google play and I don't understand why.