Hey there gretanachajova,
Thanks for reaching out to the Apple Support Communities. For the best steps that can help with troubleshooting issues with signing into iCloud, check out this link here: If you can't connect or sign in to iCloud - Apple Support
If still having issues after going through the steps above, test the issue on another web browser if possible.
Keep us updated on the results, and let us know which device and software version you're using when trying to sign in, and we'll continue from there.
Have a great day!