iCloud Problem with El Capitan

Hi! I cant log in to my icloud account and itunes as well. Already went to the Power Mac Center and they told me that they need to downgrade my OS from El Capitan to Mavericks since it's not compatible with my Mac anymore. But my Mac is Pro Mid 2012.

Now, I'm having a hard time signing in to my accounts. Every time I try signing in itunes it says .

Same with my icloud, it says

Please help me with this. I tried checking my keychain access but there's nothing in there.

I really don't know what to do. Thank you for reading this 😟 🙂