Error Message while rebooting Nexus 7 tablet for ubuntu touch

I tried installing Ubuntu touch on my nexus 7 wifi tablet 2013 edition and I am getting this error:

"Your phone needs restoring from a PC or service center"

could anyone please tell me how to make it? Do i have to install android again and then try or should i go back in a bootloader mode and flash it again. In the end, some menus comes on the screen and it stays longer so we need to select it using CWM. Do i need to give a hard reset for reboot? Please suggest me on this. Thanks in advance!!!