I have connected my eBay account using eBay app integration and it was successful. In the apps sections it still shows connected but when I open it, it doesn't open.

The apps connected via OneSaas have been discontinued, which is why you're seeing an error message even though the connection still appears to be active, Awais.
 
As eBay is being migrated from the OneSaas integration tool to QuickBooks (QB), you will receive an email when your app is ready for the transition. This notification will include detailed instructions on what to do.
 
In the meantime, you can import your data using a CSV file or enter your transactions manually into QuickBooks.

After your eBay app has moved to QB, you can check out the bottom section of this article for the steps on how to access it: App integrations.
 
If you have further questions, comment below.