This generally only occurs when your Store Name is either a generic naming or already matches an existing store. 

NOTE: Make sure the app is confirmed as uninstalled at this point

To fix this, start by navigating to your Store Setup in BigCommerce:

Then go to Store Profile and change the Store Name to something unique:

Once you have made this change, wait up to 15 to 20 minutes and then reinstall the app once more.

If you are still experiencing an install issue, let our team know.