Troubleshoot iCloud Error 18003
                     Troubleshoot iCloud Error 18003
Our page will be a complete guide to fix your 
iCloud error 18003 easily. Proceed with the 
steps below to fix iTunes on your Mac 
computer/laptop as follows.
MacOS Mojave Or Later
Step 1:
On the home screen of your Mac computer, navigate to the Apple 
menu and select the System Preferences option.
Step 2:
Click the Software Update button.
Step 3:
If an update is available for your Mac, you will see the Update Now 
button. .
Step 4:
Click it to update the Mac computer to the most recent version
Step 5:
Select the ‘Automatically keep my Mac up to date’ option.
MacOS High Sierra Or Earlier
Step 1:
First, launch the App Store application on your Mac computer.
Step 2:
Click the Updates tab at the top of the screen and select the Update 
All option.
Step 3:
If this option is not visible, the system is up to date.
Step 4:
To download updates automatically, go to the Apple menu and select 
System Preferences -> App Store -> Download newly available 
updates in the background to fix iCloud error 18003.
REFERENCE : https://mail-setup.com/icloud-error-18003