Hi bente129,
Thanks for reaching out to Apple Support Communities. It sounds like you're having an issue with adding your iCloud account to your Mac. We're happy to help.
Are you able to add your account or sign into iCloud from System Preferences? If you're unsure, from the Apple menus choose System Preferences and based on your macOS do one of the following in the section "Set up a Mac" in the guide here: Set up iCloud Mail on your devices
We're also seeing an outage was recently resolved. System Status If you continue to have any issues, confirm the macOS you're using and how your Mac is connected to the internet, we'll take it from there.
Cheers!