The standalone Grammarly Keyboard app for Android was discontinued and phased out around early-to-mid 2024. It was replaced by a new, more integrated "Grammarly for Android" app, which acts as an assistant that works with any keyboard rather than a dedicated keyboard app itself.
Uninstall the old dedicated keyboard and install the new "Grammarly for Android" from the Google Play Store. It now works as a overlaying widget.
