A little bit of background here. I previously had High Sierra installed on my Macbook Air, but for some reason, at one point when I tried to boot up my mac, it showed a folder with a question mark sign that kept flashing. Luckily for me, I was about to upgrade to macOS Mojave, so I had a bootable disk with Mojave on it. I created a new volume on the existing Container in recovery mode, installed Mojave on it and copied out files from the corrupted High Sierra Volume.
Now, I had previously read on some articles and forums that the way to move keychains manually to a new mac was to navigate to ~/Library/Keychains/ and copy out the login keychain, but I found out that Wifi passwords were stored on the System keychain in Keychain Access, so I navigated to /Library/Keychains/ and copied out System.keychain along with the login keychain from the user folder and I deleted the volume.
Coming to my Mojave and opening up the High Sierra's System.keychain in Keychain access, I found that I was unable to copy the Airport passwords to my main Mojave System keychain because it was requesting for the High Sierra's System.keychain password. This is puzzling, because I already was able to unlock the High Sierra's System.keychain with my Mojave user password which by the way is the same as my previous High Sierra user password.
Yet, when I select all the Airport passwords in the High Sierra's System.keychain and try to paste them in Mojave's System keychain, I get shown this: 
Typing in the correct password doesn't work, the pop up window just jiggles to show that the password is incorrect. I don't know what else to do, as I have quite a lot of Wifi passwords. Please, any help would be appreciated.
Best Answer
I believe that in Mojave you cannot simply copy the keychain file. I found the best solution to be to sync the keychain in iCloud. This assumes that you have access to both machines.
Go to system preferences -> ICloud. Select Keychain.
Repeat this on the second machine.
Your keychains should now be transferred.