Can you give us a little more information? For example - can you go and look at the logs and find the full backtrace of the error that is occurring?
Otherwise we don't know where the error is occurring. Then we can narrow it down a bit better
My best guess atm, though, is that you have the wrong version of ruby or rails or rake (or something similarly important). mattr_acessor is a fundamental method in ruby - so I'd start by reading the redmine docs to see what version of ruby you need.
Then go check if you've got that version installed... repeat with rails, required gems etc.