Why will my interior thumb turn only turn if the key is inserted into the exterior?

There is an installation error, you need to uninstall the lock and reinstall the lock. During installation ensure the key is not in the cylinder and the interior thumb turn will now function without the key in the cylinder.

DISCLAIMER:

INFORMATION PROVIDED THROUGH THIS SITE IS PROVIDED TO YOU AS IS WITHOUT ANY EXPRESS REPRESENTATIONS OR WARRANTIES OF ANY KIND, AND WE MAKE NO REPRESENTATION OR WARRANTY THAT THIS SITE(OR ANY INFORMATION PROVIDED IN RESPONSE TO YOUR INQUIRY), WILL BE ACCURATE, COMPLETE, OR ERROR-FREE.

YOU AGREE THAT YOU MUST EVALUATE ALL INFORMATION AND RESPONSES, AND THAT YOU BEAR ALL RISKS ASSOCIATED WITH, THE USE OF THIS SITE, INCLUDING ANY RELIANCE ON THE ACCURACY,COMPLETENESS, OR USEFULNESS OF ANY INFORMATION OR MATERIALS MADE AVAILABLE THROUGH THIS SITE.