Hi Ilyabrykau,
It seems you're having trouble enabling the option to Mirror Front Camera, and we can help.
Are you running the release version of iOS 14?
Go to Settings > General > About > Software Version, and tap on Software Version. Tell us what you see there.
Now check Settings > Camera > Composition > and toggle Mirror Front Camera.
If you do not see this setting, keep in mind that Mirror Front Camera is available on iPhone XS, iPhone XR, and later.
About the Camera features on your iPhone
Thanks for using the Apple Support Communities.