If you are not able to challenge your friends after updating 8 Ball Pool to the latest version, it might mean your friend is still playing on an older version of the app.
Please make sure you both are playing on the latest one! You can check what version you are currently on by accessing your Settings and scrolling down to the bottom of the page as you can see below! If both show the same Version, then you are good to go!
Also, bear in mind that mobile users are only able to challenge other mobile users. Same will apply to web users :)