Nothing happens when I press print or preview for a selected invoice, can anyone tell me why??

Welcome to the Community, Saxon.

There isn't any reported incident with the inability to print or preview invoices in QuickBooks Online. Let's perform some basic troubleshooting steps to isolate the behavior encountered in your QuickBooks Online account. To start with, use a new incognito window or a different supported browser. Using a new incognito window does not store any data file.

Here's how:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Safari: Command + Shift + N

Then, log in to QuickBooks Online account and print/preview once more your invoice. If that works, let's go back to your default browser and clear cache and cookies. 

If the issue persists, please use another supported browser. And I recommend also checking if you have an updated version of your browser.

Post again in the Community if you have further questions about printing transactions in QBO.