Intuit disabled the feature sometime back where you could connect apps to your client QBO files directly from within QBO.
Only the Admin of the App can access Apps from within QuickBooks Online.
You have 2 options now to do this:
- Go to www.apps.com and login from there to connect
- Start at the app website and work backwards. Login to the app, go through the Connect to QBO steps and then using your QBOA login, you can choose which QBO client you want to connect the app to.
If you are the Admin user and still cannot see the App then it is possible that the Intuit access token has expired. You can go to Apps.com and search for the App and select try it now and the reconnect the App.