In portrait mode front camera not working

What model iPhone do you own? Portrait mode using the selfie camera is available on iPhone X or newer.

See this link --> Use Portrait mode on your iPhone - Apple Support

Portrait mode should be available on iPhone Xs. Have you had any service done on your phone?

Try a restart and if that doesn't help, you may need to schedule an appointment at your Apple Store Genius Bar for evaluation and options:

Restart your iPhone, iPad, or iPod touch. 

You're welcome and good luck