The SD card needs to be formatted to the primary FAT32 (or exFAT) MBR partition type.
Polyend Play is offered with a 16 GB microSD card, larger SD cards can also be used as long as they are correctly formatted. We recommend UHS-I cards, particularly ones that are rated A1 or A2. The built-in disk-utility under Mac OS will allow you to select FAT32 for formatting.