Do I need to learn how to use Reaper?
No, you don’t need to learn how to use Reaper to use it with Soft Arranger. You just need to install it, set set a few preferences which are clearly explained in the set up instructions and then load a preset which is included with the Soft Arranger download files. After that Reaper will just run in the background and you don’t need to do anything else with it.
Of course Reaper is an awesome DAW which allows you to tweak your sound, record your performance along with other instruments or singers and carry out edits to an extent which is not possible with any hardware arranger costing thousands of dollars so you might want to play around with it and learn how to use it but that is totally up to you.