Q: Keychain password

Originally, the keychain password is the same as the login password for the account. The login password can be changed in any of three ways:

1. By the user, in the Users & Groups preference pane. In this case, the keychain password is automatically changed to the new login password
2. By the administrator (if different), also in the preference pane. The keychain password is not changed.
3. By anyone who has physical access to the computer, in Recovery mode. The keychain is replaced by a new, empty one with the new password, and the old keychain is saved with the old password.

To change a keychain password, please launch the Keychain Access application and select the keychain from the list on the left. If the list isn't showing, select

          View ▹ Show Keychains

from the menu bar, or press the key combination command-K. Then select

          Edit ▹ Change Password for Keychain...

You must know the old password in order to set a new one. Otherwise you'll have to reset the keychain.

Do you have more than one user keychain, such as "login" and another one with your name? Do you use iCloud Keychain?

Please take each of the following steps that you haven't already tried, testing after each one. Back up all data before making any changes.

Step 1

Open the iCloud preference pane and uncheck the Keychain box. You'll be prompted to delete the local iCloud keychain. Confirm. Then re-check the box. Follow one of the procedures described in this support article to set up iCloud Keychain on an additional device.

Step 2

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. There should be a file in that folder with the name "login.keychain". If there is also a file iwith the name "login_renamed_1.keychain", then please do as follows:

☞ Rename login.keychain to "login-old.keychain".

☞ Rename login_renamed_1.keychain to "login.keychain".

You can then close the folder. 

Delete the login keychain from the keychain list in Keychain Access. Choose

           Delete References

when prompted, not Delete References & Files.

Select

          File ▹ Add Keychain...

from the menu bar. Add back the file now named "login.keychain". If any of your needed keychain items are missing from it, also add back the file now named "login-old.keychain". I suggest you transfer any needed items from that keychain to the login keychain, then delete it. The transfers are made by drag-and-drop in Keychain Access. You'll need to enter your password for each item transferred.

Quit Keychain Access.

Step 3

If you still have problems, uncheck the Keychain box again in the iCloud preference pane and continue.

Inside the Keychains folder that you opened in Step 2 is a subfolder 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 the subfolder (not the Keychains folder) to the Trash.

Restart the computer, empty the Trash, and re-enable iCloud Keychain.

Follow an apple article : If your Mac keeps asking for the login keychain password - Apple Support

What is the exact, complete text of the password prompt? Do you see it only once after logging in?