Why won't my Motorola 6.0.1 let me use my SD card?

I understand my phone has a lot of memory on it a lot of gigabyte memory but I don't think that it's right that I can't use the SD card to free up space so I can download more things, and it's really frustrating because there's a slot for an SD card but it won't recognize it, it won't move anything. Facebook and the like are pretty big apps and they're just taking up space on my phone not to mention all the space my phone alone takes! help me please

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