I flashed this one (TWRP 3.0) https://drive.google.com/uc?export=download&id=0B9O4F5W7jWBlcE9tek5GSVpwVFkCan't help you there, you'll need to wait that someone answer your on that problem. Meanwhile search around on net, you aren't only one with that issue.
Which twrp you have, did you installed from link provided here?
Maybe you'll need different twrp version to have installed, one with possibilities to format encrypted partition.
Try to install latest twrp for your device it's seems you have older twrp. Latest 3.0.2.1 have possibilities to format encrypted partition.
Search it on net, but install version which is for your device not any twrp
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I thought maybe it was because of that but I don't really know which one I should flash for m'y RN3P, does anyone have a link for the most recent version ?