Unable to sign into iCloud/itunes after installing el capitan

After trying tons of different things, your method finally worked! Thanks a lot!

awasthigenicus solution was a stroke of genius.

I was able to log in App store in my mac

Thanks! This worked for me as well.

After updating to El Capitan, I lost the ability to log into my Apple ID through iTunes. Nothing would happen when I tried to login from the store menu or the top of the main iTunes window. The App Store and everything else that requires Apple ID worked fine but no luck in iTunes proper. This made syncing apps, making and transferring purchases impossible. I did my homework on the issue; no serial number problems, killing processes didn't help. The solution that has been mentioned elsewhere, that also worked for me, was to reinstall El Capitan. Fortunately, this is a very simple process and took less than 30 minutes to complete.

Boot from the Recovery Partition (hold down command - R on startup)

Select Reinstall OS X from the list of options

That's all there was to it. Everything else on the disk remains untouched, Documents, Music, Etc.

When I started iTunes, I clicked on Sign In, and was able to do so with no issue.

Awesome. Thanks! No other suggested solution worked so far but this hit the sweet spot.

THANK YOU!!!!

This didn't work for me at all. I must be the only one, but I'm in the same boat as I was before. I followed the suggestion with terminal exactly as written, but I still can't get past the ****ing sign-in screen. And on a brand-new Macbook, too. I just opened the box this morning. :/ Any other suggestions? I DO have a serial number.

The funny-not-funny thing is that I just installed El Capitan on my old laptop and I have zero sign-in issyes. So I don't get it.