Please take these steps to resynchronize the iCloud keychain. Your keychain on iCloud and your other Apple devices won't be affected. Take Step 2 only if Step 1 doesn't solve the problem.
iCloud Keychain has rather complex security safeguards that can be tricky to work with. It also has its fair share of bugs. If you have doubts about whether you can carry out the instructions below, it may be best to contact Apple Support before doing anything.
Step 1
Back up all data.
Open the iCloud pane in System Preferences and uncheck the Keychain box. You'll be prompted to delete the local iCloud keychain. Confirm—the data will remain on the servers. Then re-check the box. Follow one of the procedures described in this support article to set up iCloud Keychain on an additional device. Test.
If you can't disable iCloud Keychain because the box re-checks itself, disable it on all your other devices first, then try again. Before you do that, make sure you can use at least one of the methods given in the support article linked above, under the heading "How do I set up iCloud Keychain on a new device without another device to approve from?"
Step 2
If you still have problems, uncheck the Keychain box again and continue.
Triple-click the line below on this page to select it, then copy the text to the Clipboard by pressing the key combination command-C:
~/Library/Keychains
In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.
A folder named "Keychains" should open. Inside it are one or more subfolders, each with a long name similar to (but not the same as) this:
421DE5CA-D745-3AC1-91B0-CE5FC0ABA128
The above is only an example; yours will have a different name of the same general form. Drag those subfolders (not the Keychains folder) to the Trash.
Restart the computer, empty the Trash, and re-enable iCloud Keychain.
