Where's my storage gone? (insufficient storage available android)

I recently bought a new phone from wish.com, I installed one app it was fine but when I wanted to install anthor app it said insufficient storage available but I had 6.5gb free! And I also had a 16gb sd card and didn't help either ; cleared the cache from the settings but that did not help but when I deleted an app that I installed perviously and installed a cache cleaner it said that I had 23mb/101mb available! I don't know what to do about this so please help me ASAP thanks
Ps im also running android 5.1 lolipop

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