Greetings Endatronix,
Thanks for using the Apple Support Communities!
I understand that you are unable to download a purchased song from iTunes on your iPhone 6 and receive the error you describe (and show) in your question. We know that situations like this can be frustrating, but I do have several suggestions for you.
I would first check to see if the download was interrupted. The following article will walk you through this process.
Resume interrupted iTunes Store downloads - Apple Support
If the songs still do not download, I would suggest trying the download on a different network. For example, if you are using the cellular network, you could switch to Wi-Fi, or you could try an alternate Wi-Fi network.
Download your past purchases - Apple Support
If you still are unable to download these songs, you can report the issue with the purchases to iTunes Support using the steps in the next article.
Report a problem with an item purchased from the iTunes Store, App Store, Mac App Store, or iBooks Store - Apple Support
Have a great day!