I get the error Apple Pay has been disabled because the security settings of this Mac were modified. How can I change them?

Follow all these instructions: If Apple Pay on your Mac is turned off because security settings were modified - Apple Support.

"If you still get the same Apple Pay alert after installing all available software updates and restarting your Mac, contact Apple Support for help."

That's the reason for Apple's conclusion:

"If you still get the same Apple Pay alert after installing all available software updates and restarting your Mac, contact Apple Support for help."

To contact Apple Support, click the highlighed link.