Portrait mode is not working on my iPhone 8 Plus, I’ve updated the software. What could be the problem?

When you say it doesn't focus, are you referring to the bokeh effect of blurring the background? This support document explains the use of Portrait mode, however there is no troubleshooting steps there. If you are following the steps in the support document and it is not working the way you expect, you can use the Contact Support link at the top of the page here to contact Apple Support for additional assistance. Use Portrait mode on your iPhone - Apple Support

"Not working" is rather vague. What is it not doing? Can you switch to Portrait Mode? Are you seeing any warning or messages on the screen when you attempt to take a photo? Describe the issue and what you have tried besides updating the software.