We use Clover for our POS system. It has an accounting sync button that should connect to QB's online, but it won't connect. Any suggestions?

As I know Clover doesn't support QB natively. You need a 3rd party connector.

Welcome to the Community space, jpukys-midlake. I'll be happy to steer you in the right direction so you can integrate your POS system into QuickBooks. 

When connecting Clover to QuickBooks Online, I recommend contacting Clover's Support Team for assistance. They're the best ones that can guide you with the linking process. You can also ask for help regarding the issue you've encountered with the accounting sync button from the third-party app.

See this page for the details on how this app works: Clover by Commerce Sync.

I'm always here to help if you have any other concerns related to QuickBooks tasks. Kindly tag my name in the comment section, and I'll get back to you as soon as possible.