First, I made sure on the Apple site that I could sign into the iTunes Store to make sure I was using the right password for iTunes and it was. However, when I went to redeem a gift card on iTunes, the box kept popping up asking me to enter my password over and over again and I could not get past it. I am on my MacBook Pro, OSX 10.6.8 and my iTunes version is 11.1.3. I shut down my laptop, restarted, made sure in Safari pop ups were not blocked, private browsing off and preferences set to Never deny cookies and I emptied the cache. I forced quit iTunes and then relaunched it. This worked.