I am unable to force quit my iphone

As i can see you have iphone xr , sometimes iphone with full screens are kinda hard to force quit you have to be very precise while pressing buttons on of iphone.

follow the steps-

Quickly press and release the Volume Up button.

Quickly press and release the Volume Down button

Finally, press and hold the side button until the Apple logo appears, and then release.

• Restart the device. That's it.
• Still not solved --> Force restart iPhone - Apple Support

Do you want to force quit an app?

How to close an app

1. From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.
2. Swipe right or left to find the app that you want to close.
3. Swipe up on the app's preview to close the app.

Close an app on your iPhone or iPod touch - Apple Support