It is not uncommon that after updating 8 Ball Pool to the latest version you are not able to challenge your friends, it might mean your friend is still playing on an older version of the app.
Always make sure you both are playing on the same game version!
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! When a new update is released, sometimes it's not released for everyone at the same time, due to a variety of factors.
This means that sometimes you might have to wait a bit longer before being able to challenge your favorite friend!
Also, bear in mind that mobile users are only able to challenge other mobile users. Same will apply to web users.
We hope this was helpful, and wish you good luck with your next match!