You installed Request Tracker 3.8 on Ubuntu 10.04 using apt-get, aptitude, or synaptic, and then you needed RT::Authen::ExternalAuth.
What you naturally did is:
sudo cpan -i RT::Authen::ExternalAuth
But you will get the following error:
prerequisite RT 0 not found
This is because you did not install request-tracker through CPAN.
An easy solution for this is to force the installation using the "-f" flag:
sudo cpan -fi RT::Authen::ExternalAuth