Error on installation of Ruby On Rails on QNAP TS809U-RP [Linux]

You need to install the development tools. On Ubuntu this would be the package build-essential, plus some others. I don't know whether it will help but googling for

install ruby rails qnap

found this [1] and others

If possible I suggest that you use rvm to install RoR.

Colin

[1] [Tutorial] Ruby 1.9.2, Rails 3.1 on QNAP 219P+ - QNAP NAS Community Forum