THE OPERATION COULD NOT COMPLETED ON IPHONE

You can try a few things--

First, try turning the device off and back on.

If that doesn't work, do a hard reset by holding the top button and home key until the apple appears, then release. This won't erase anything, but may help.

Lastly, I would check your settings, that you're on and connected to wifi, close to the router, and have the newest operating system update.

I had this "Operation could not be completed" problem with my Pinterest app. A button on my iPhone 5 broke and I got a new phone. Everything was backed up, but I still had to log into some of the apps. I did all of the above and still couldn't get it to work. I figured out that it was because I normally log into Pinterest through Facebook and the Facebook app wasn't installed yet. Once that was set up by entering my user name and password on FB, I had to save the password in my settings > facebook. Once that was done, I was able to log into Pinterest without any problems.

Hope that helps!