Oneplus added a feature to use same app multiple times. Called parallel apps which is much better than using a 3rd party app.
Do a full wipe factory reset after taking backup.
Clean your camera
Are you on stable version?
Clear Cache and reboot the phone.
Download offline Ota and flash it.
Do you get some notifications on screen of just vibrations?
Get the offline Ota files update from mega thead and flash the same manually.
Which version you are on?
Seems you hard bricked the phone.. Try flashing the official recovery first and see something appears.
Yeah i agree the fact its a sales strategy.
Actually they removed due to security reasons.
Its not working any more they removed that function.