I had a beautifully working 2.6.6 and upgraded but now i cant create a new item in front end or backend.
Please advise. I saw this issue for older posts but all were fixed by updating. But updating caused this. I updated with just the "update" zip found in the google code page but then also again with the full download install.

Update: I reverted back to 2.6.7 so the site can function and ill do my testing a backup site (rookie mistake ).

But i did hide the toggle switch so the calendar and meta info etc wouldnt show on frontend. So technicially i do have the calendar hidden. But in the backend i havent touched the form so it should load for 2.6.7 just as it does in 2.6.6 right?

I have found this error before, and tried this fix:
Edit the file: libraries/joomla/html/html.php

Go to the line ~879 and find this line:
And replace it with this line:
Save the file and test.

This fix worked with me, hope it works with you as well

still i am getting error after changes.
K2 2.6.7 have already this change so no need to change so please give me another solution if any idea
thanks in advance

Try to upgrade to K2 SVN 2.6.8 version located here:http://getk2.org/assets/get/