Solved: Fix for "Apple Pay has been disabled because the security settings of this Mac were modified".

A restart was necessary for me as well. To be perfectly clear, here’s what I did:

• Open System Prefs → Software Update
• Click Advanced…
• Uncheck “Install system data files and security updates“ (all others checked)
• A dialog appears asking me to authorize w/Touch ID; do that
• Reboot
• System Prefs → Software Update → Advanced… again
• Recheck “Install system data files and security updates“ (all others still checked)
• Go to System Prefs → Wallet & Apple Pay
• Previously authorized cards now available again

Not sure if you have managed to resolve this since posting this, but I've had the same issue, nothing would resolve this issue. To save you spending hours on the phone like I did follow these steps

1. Ensure that your 'FIND MY' is turned off on your Mac
2. Turn your Mac off
3. Turn Mac back on and just before the apple logo appears hold 'COMMAND' and 'R'
4. This will take you to the MacOS Recovery screen
5. Log in to your account when it appears
6. Once logged on, go to the very top bar and select utilities
7. Click the startup security utility
8. This should then ask you for an administrators password, just your log in password as normal.
9. If your 'FIND MY' isn't turned off you won't be able to enter administrator's password
10. Once you are on that you need to select the highest security setting.
11. It will then give you a box to restart your Mac
12. Turn it back on as normal and try from there

Hopefully this will resolve it for you. I purchased my Mac secondhand and the previous owner had the security settings set to 'none', causing me the issues with adding my card.

This worked for me but I had to restart my laptop after unchecking the “install system data files and security updates” box and then check it again after it turned back on. Only then I was able to setup Apple Pay.

This worked for me as well. For me, though, the setting was disabled for some reason. I expect it might be due to the fact that I bought a new 16" MBP, and my old 15" 2015 MBP did not have the T2 security chip (or whatever it's called). So, if anyone is upgrading from an older model like me, you might find yourself in a similar situation.

Thanks for the tip, I would not have thought to look there!

Thank you for posting this information. There have been a few cases of this and so far this is the solution. I added this information to a User Tip I created for Apple Pay issues - Problems and their solutions for Apple Pa… - Apple Community

Thank you. Apple support was no use on solving this for me. It worked just as you said. Thank you again

Thank you. Totally solved that sticky ApplyPay issue on my Air.

I had "Install system data files and security updates" on my previous MacBook, and I guess that setting got transferred to my new MacBook. I had Apple Pay working on the old MacBook just fine, however, even with that option unchecked.

This thread was great for giving me the hint to try this Software Update setting, because the page that Apple links you to when Apple Pay is disallowed says nothing about this issue, and I never would have guessed to try "Install system data files and security updates" otherwise.

Perhaps its just a matter of whether it's checked at the time you add credit cards to your Mac, and then you can turn it off later and you'll still be okay?

While I like to keep MacOS up-to-date, I only want automatic downloads and notifications — I definitely don't like any setup that automatically shuts down and reboots my system when I might have been in the middle of something I don't want interrupted.

Great!! That really worked... would have never thought of this (and also have now clue why it works).

I was also moving to a new MacBook Pro and had my complete volume cloned. Now the cards are already setup!

Thanks alot.

I can't thank you enough. I have spent all morning running through impossible commands and procedures that were recommended to solve this problem. I read your post and followed your instruction and it fixed everything within a few minutes. Thank you so much!!!!!

Thanks!!! I've been looking everywhere to figure this out. That box was unchecked on my Mac. So I checked it and went back to Wallet, clicked Add Card, and - voila - all my cards from my iPhone magically appeared. I was able to setup them up after verifying each. Apple Support did not even provide this resolution!

Thanks so much...

Thank you so much - mine was already unchecked so I checked it unchecked it then checked it again and it worked - so I unchecked it and it stopped working - so you must have it checked for Apple Pay to work.

Thank you so much, that solution also worked for me. I have a new MacBook Air and like you I was trying everything. I'm so pleased it's resolved thank you. Linda