Hello RLee0603,
To help us better assist you, can you let us know if that error appears when you insert your SD card, or if it appears only after trying to reformat the card? Do you experience issues with any other SD cards?
Those details will help us isolate the issue.
Please also check out this link, which has some information on using SD cards with Mac, and information how to resolve some issues, which may help: Use the SD and SDXC card slot on your Mac
"If your card doesn’t mount
If you insert your card and it doesn’t mount, remove the card and insert it again. Sometimes the SD card won’t mount properly if you insert the card too slowly or too quickly.
If the card still won't mount, you might need to reformat your SD card.
If 'cannot be modified' appears
If the message 'cannot be modified' appears when trying to use the card, the SD card might be locked.
To unlock the card, eject the card, then remove the card from the computer. Adjust the lock slider tab on the card to unlock the card, then reinsert the card into the slot. See the manufacturer’s instructions for the location of the slider tab."
Thank you!