Default Can't Wipe Internal Storage, Running on Stock (XT912)

So I've been wanting to wipe my internal storage to help free up space on my phones RAM and I'm running PAC-ROM on my phones stock slot. Is it because I'm stock slot? I really wanna clear up some space and doing it manually from my.phones file manger ain't the easiest way. Everyyime I'm in SSv3.65 and go to wipe Internal it says *Unable to wipe /sdcard*..so can anyone help me?

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