Solution:
1. Go to change Hide App Passcode to try again.
realme UI 2.0 & above versions: Settings > Search "Hide apps" in the search bar > Hide apps > Settings on the top right > change access code.
2. Reset your device system settings to the default values to try again.
realme UI 3.0 & above versions: Settings > System/Additional Settings/ System & updates > Back up and reset > Reset Phone > Reset all settings.
realme UI 2.0: Settings > Additional Settings > Backup & Reset > Erase all data(factory reset) > Reset system settings only.
3. Restart your device to try again.
4. Also, make sure your device is running on the latest version of the software. You can check for updates in Settings. Updating your device can help resolve many common problems.
Note: The Hide App feature works along with Google Dialer only. If you are using any other dialer, please switch back to Google dialer to use the Hide App feature.