Error After Login Admin Panel

Hi,

I am new in this forum and this is my first post.

Previously I am using V1.5.6 and just upgraded to OpenCart 2.0. Everything works fine from step 1 until Step 7 as stated in Upgrade Note.

Now I have these two error line when I go to the admin panel dlz.biz/admin

Notice: Use of undefined constant DIR_MODIFICATION - assumed 'DIR_MODIFICATION' in
/home/dlaz/public_html/system/startup.php on line 53

Notice: Use of undefined constant DIR_MODIFICATION - assumed 'DIR_MODIFICATION' in /home/dlaz/public_html/system/startup.php on line 57

These 2 lines repeated for 12X and the login section appears finally at the bottom. I tried to proceed by
entering the admin username and password, then these two line came up again and it stopped there.

Please advice me how to proceed or else I have to revert to the old version.

Thanks