The following procedure is for android 9. If you are using a different OS version, the procedure may be different.
First, You need to allow the Virtual Key to overlay other apps.
1. Tap OK.
2. Switch "Arrow permission" to ON.
Secondary, You need to allow the Virtual Key to has Accessibility permission.
1. Tap OK.
2. Tap "Installed services".
3. Tap "Virtual Key".
4. Switch to ON. Then "Key" is appered.