The error message is not very helpful, here. Be sure you uploaded all of the cgi, Modules, and Templates, and that all files except graphics were uploaded in ascii mode. Upload ubb6_test.cgi from the help directory of the zip to your cgi directory and run it from your browser.
Check the Perl path in cp.cgi and ultimatebb.cgi to be sure they are correct for Perl - version 5. Recheck permissions on all files per instructions.
The current version of UBB6 is 6.05. That is what you should be installing. It is in the Member area.
If this does not clear your problem, fill out a
trouble ticket so an Infopop tech can take a look.