Hi, Ioannis,
Thanks for visiting the Apple Support Communities. From your post, I understand you're having trouble removing a friend from Find My Friends. When you try, you get an error that the server can't be reached. I'm happy to help!
The error you're receiving indicates to me that there may be some sort of connectivity issue between your device(s) and the iCloud servers. If this is there are network related issues, or an outage with iCloud and its services, you may have trouble performing certain functions. There could also be issues with the network you're connected to.
To troubleshoot the possible connection issue, I recommend the steps in If you can't connect to the App Store, iTunes Store, or Apple Books - Apple Support, which not only help with iTunes and App store connectivity (as the title suggests), but also issues connecting to other Apple services (like iCloud and Find My Friends).
Have a good day.