The -12 failure code indicates INSTALL_FAILED_OLDER_SDK, meaning that the app was built for - and requires - a version of Android that is newer than the version you have on your device (source). For example, you should get this error if you had an Android 1.6 device and tried to install an app which required Android 2.1 (or higher). You would need to either:
Update your device to a newer version of Android somehow, OR
Ask the app developer if it's possible to build the app for an older version of Android.