Try some of these “fixes” below if:
Best long term fix is to purchase a new microSD card that works with the printer. I and others have had good luck with this Kingston 2 GB microSD Flash Memory Card SDC/2GBSP (Single Pack) from Amazon.
I purchased the card from Amazon seller “i77e”. It should not matter which seller to purchase from but I wanted to share in case it does.
If printing via microSD card: Are you using the included microSD card? If so the files are known to get corrupted. If you open the file from the microSD card in a text editor such as Notepad++ or Sublime Text you may find a lot of U's further down in the gcode file.
Format the microSD card. If using Windows uncheck the Quick Format box. It is best to perform a full format to scan the card for bad sectors.
Are you using Cura? If so make sure you save the gcode file from Cura to the hard drive and then from the hard drive to the microSD card.
Try other microSD cards. Try any card up to 32GB (yes some 32GB cards will work with the MP Select Mini V2 and the MP Mini Delta). FAT16, and FAT32 both work. Also exFAT works for some. The printer is picky about which cards is will read so if you have multiple microSD cards try them all.
You may also want to try formatting the larger cards such as 16GB and 32GB with different allocation unit/block sizes.
You can try the same with the lower capacity cards also but using smaller allocation unit/ block sizes. Also try FAT(16) with different allocation unit/ block sizes.