If you successfully went and connected a data source to Sparrow, you should see one or many connected accounts in the dropdown in the connection modal:
Sometimes, an account might be missing from that list. Here are some common reasons for this:
Do not have appropriate permissions/access
If you're not the admin of the account you are missing, then it's likely a permissions issue that is causing Sparrow not to show up. Verify with your account admin that you have access to analytics or reporting permissions.
Gained access to the page after Sparrow connection
If the page you are missing is new, it's likely you just need to refresh your authorization connection to see the new page. This can be done by going through the Connect flow again:
You should then see the new account in the list of available accounts upon successful reauthorization. Here's an article with a more in-depth guide for completing this task for Facebook and Instagram.
Account/Page never authorized to be connected
With some sources, you have the option to connect multiple pages/accounts during the same authorization flow (like with Facebook or Instagram). If you know you have administrative access to your pages, but you're not seeing them in Sparrow, then it's likely you did not approve access to the page while going through the authorization flow on the connector's site.
This issue can be resolved by going through the authorization flow again and making sure you enable Sparrow to have access to the accounts/pages you want to connect to Sparrow.
Here's an article walking through this for Facebook and Instagram.