Apple Pay has been disabled because the security settings of this Mac were modified.

If none of the suggested steps shown on Apple's support page seem to solve the issue, such as making sure the notebook lid is open, checking for the latest software and making sure the tick mark for "install system data files and security updates is checked, and checking to make sure the startup disk is being launched at full security, you can see if your MacBook has System Integrity Protection enabled. To check this, locate your System Informations application and launch it. From there, select the sidebar that says Software. From the System Software Overview, look to see if System Integrity Protection is enabled. If not, locate your terminal application and launch. Enter the code, "csrutil enable" into terminal and press return. Hopefully this will solve the issue. Terminal could also return a message saying this action cannot be executed unless done in Recovery OS.

If needed to go into Recovery OS, it is no problem, it's easy. From here, shut down your computer and turn back on again as usual, except this time, hold down Command-R immediately after seeing the Apple logo and continue to hold until the MacBook is launched in Recovery mode. Login as usual and locate terminal from the menu bar above. Enter the same code, "csrutil enable" and press return. Now restart the MacBook to make the changes go into effect. Hopefully this solves the issue for you.