Apple ID is "temporarily locked" signing into iCloud on Mac

Facts:

I am donating an old MacBook Air to a family member. She is an Apple user and has an iPhone and an iPad, both of which are on the same iCloud / Apple ID account, and both of which are running iOS 9. She uses two factor authentication (not two step).

I did a clean install of El Capitan on the MacBook Air. When it came to setting up iCloud in the set up sequence it asked me for the password, which was fine, and then to approve the MacBook Air sign in on another device. The approval request didn't arrive on either device. Then there was a 4 digit passcode, and I used the only one I know to be used on her devices, but this didn't work, and on the third try it locked the iCloud account.

I skipped this step and tried to log in to iCloud in System Preferences, but each time I get an error message saying:

"Your Apple ID has been temporarily locked."

"Try signing in again later"

I have signed into iCloud.com, and Apple ID, and reset the password multiple times (reset the password 4 times now). The iPhone and the iPad are fine, and continue to work normally with iCloud (from what I can see). I can also use the Mac App Store, iMessage and FaceTime on the MacBook Air, and because of this the MacBook Air is listed in trusted devices in the Apple ID settings.

Questions:

What on earth can I do to fix this?

How long does the temporary lock last?

Thank you in advance for any help,

Kind regards