I have some troubleshooting steps to share so you'll be able to print your customers transaction in QuickBooks Online for Mac.
Let's start with accessing your account using a supported browser. This can help us determine if the cache of your app is the reason behind this issue.
If you're able to print the customer's transaction from the browser, let's go back to the app and have it refreshed the clear its cache afterward. Doing this deletes the stored data which causes problems in the program and refreshes the application.
To do that:
Click Help on the navigation bar at the top.
Select Reset App Data.
Allow a moment for the process to complete, and then close the app.
Be sure to right-click the app and Run as Administrator when re-opening.
Once done, go to File in the upper left corner, and then Clear Cache