Have installed Ubuntu Nexus 7 Desktop Installer
Have Unlocked the Nexus 7
Nexus in fast boot
have searched for related questions and google
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You could try the Manual steps, also on the Installation Instructions page.
Wiki > Setting up your Nexus 7 > Manually Installing Ubuntu 13.04 on Nexus 7
If several retries with the 'tool' have not succeeded, this may show more clearly what is not working (or failing) ..