Hello,
when I am trying to create a new account, a warning message appears on the top:
Empty solution not allowed.

I feel all the required areas but the message still appears.
Can you help me please?

I had this problem too.
the answer of this quastion is here:
getk2.org/community/New-to-K2-Ask-here-first/10508-Re-reCaptcha-not-working#11485

you should disable recaptcha at :
administrator/users/options
and set "captcha" to : none selected