If you encounter 'unable to complete installation' error during installation, check first that you have the latest phone software installed in your phone (PR1.2). Select Settings > Applications > Manage applications > Updates to check for updates.
If the problem persists or there is no software update available for your phone, you can try the following:
- Enable the developer mode temporarily in Settings > Security > Developer mode and try the installation again. After the installation you can disable the developer mode.
- Check if you can install any other applications from Nokia Store (in which case the fault may be in the application you are trying to install).
- If the problems still continue, you can try to reset the device with Clear device command in Settings > Reset. Note that this will delete all the data from your device, including the media files and map data. Before resetting the device, take a backup and copy your media files (photos, videos, music) with Nokia Link to your PC.