Sorry to hear that you're having trouble downloading Pandora on your iPhone 6 device.
Please be aware that Pandora requires iOS 12 or higher. If you are running iOS 11 or lower, you will need to update your iOS version. If you're not sure how to update the OS on your phone, you can refer to this article for more help.
In the meantime, could you tell me a bit more about what is happening?
Are you receiving any error messages? If so, what exactly does it say?
Could you cancel the download?
What happens when you attempt to open the Pandora app?
I would recommend turning off your device and then, restarting it to see if that helps.