If when you launch ArtRage, you are asked to validate it everytime, or can only run it when connected to the internet, this is likely due to a problem with the google play store license for ArtRage. Try the following steps to correct the problem.
Uninstall ArtRage and reinstall. Make sure you are connected to wifi and launch ArtRage again to validate your license.
If this doesn’t work, save your current painting to prevent data loss, then close ArtRage completely. Clear ArtRage’s app data via the Android Application Manager in Settings.