Hi rus44,
Thank you for participating in the Apple Support Communities. We understand you're getting a pop up to agree to new iCloud Terms and Conditions. Are you not signed into your iMac with an Apple ID? If you are, then you have an iCloud account and you'll need to agree to the terms and conditions in order to get the pop up to stop happening. Keep in mind, when you have an Apple ID / iCloud account you can always choose which iCloud features you want to use by enabling or disabling them. This article explains more about this: Set up iCloud features on Mac.
If you're signed in on another device, you may be able to agree to the terms and conditions on a different device if needed.
We hope this helps.
Take care.