In general, you should install a release zip, rather than clone this repo. If you do want to use the git version directly, you need to clone into your addons folder and recursively init/update all submodules.
Quick and dirty:
cd ${WARCRAFT_DIR}/Interface/AddOns/
git clone https://github.com/Itarater/XRP.git
cd XRP/
git submodule update --init --recursive
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