How to format an SD card for Monoprice Select Mini V2?
For an SD card to work with the printer firmware Monoprice suggests to format the SD card to FAT32.

However, the Monoprice Select Mini V2 is not able to read SDHC memory cards, it is advised to use an SD card (smaller than 4&nbsp;GB) instead.

According to the Monoprice support website: "Why is my printer not reading my SD card?":


  If the SD card is not recognized on the printer or if the files are
  not reading, it could be an issue with the actual card itself. The
  first thing we recommend checking is that none of the print files on
  the SD card contain a space in their name. This shows in the printer
  as an unidentified character and can cause issues. If none of your
  prints contain spaces, we recommend reformatting your SD card.
  
  Note:  If you choose to purchase an SD card, please make sure that it
  is not labeled HC (High Capacity) as it may not be compatible with the
  printer. This means that the card must be smaller than 4GB in size.


The last part of the support page is probably applicable to your card.

Some further information can be found in What is the largest microSD card that a Monoprice Select Mini can read?, specifically this answer. From this latter answer, I quote:


  Cards between 2 GB and 32 GB might work, depending on the specifics of
  the card


Basically there are no guarantees when using large cards.