WHY won't my Galaxy Grand Prime (G530T model) recognize the micro sd card I put in it?

[Android Central Question]

So I bought this phone off of Ebay (so it's new to me) and the guy selling it claimed it was new. It looked in pristine condition and even had a screen protector already on it (a floor model maybe? Who knows.) Anyway, I just in the last few days decided to TRY and expand the memory since it claims (on the Samsung site) that the external memory can be a micro sd card up to 128 G's. Ok so, I bought a SanDisk Extreme Pro micro SDXC UHS card that also has the adapter with it. I put that card in the slot in the phone but the phone WON'T recognize it. I've clicked on the Format link and the Delete link after that and the Mount link and Unmount link (even though I DON'T understand WHAT that means or what it does) but the phone STILL DOESN'T recognize the card. One notification came up saying that the phone has no sim card in it. I don't know WHAT to think at this point. I wanted to expand the memory so I could MOVE some of the Apps I've put on the phone TO the card but STILL be able to use them when I wanted to and ADD a few other Apps that would be of use to me to the phone to use as well. And then of course, I love to take pix and save them! I also keep getting messages coming up telling me the memory is almost full and I need to DELETE files. That screws up the purpose of TRYING TO EXPAND THE MEMORY it seems. If any of you have any info. about this that's HELPFUL to me please let me know.