'The disk you inserted was not readable by this computer' can be caused by many reasons, such as a damaged file system, catalog file corruption, improper ejection of the SD card, etc.
Method 1: Do a basic troubleshooting. Check whether your SD card is encrypted; check whether the file system is supported by macOS; check whether your SD card is corrupted.
Method 2: Re-insert your SD card. Remove it from your Mac and then re-insert it to your Mac to check whether the issue has been fixed.
Method 3: Run First Aid. You can run First Aid in Disk Utility to check whether your SD card is intact. First Aid can help you to check and repair file system errors automatically on your disk.
Method 4: Format your SD card. If the above methods still can help you fix your issue, you can try to format your SD card. But don't forget to back up your data on it before. You can try to recover your data from it by using iBoysoft Data Recovery for Mac.
Method 5: Ask for help. If you worry that your improper operations will cause further damage to their hard drives, you can send your SD card to a local professional and they will give you some correct suggestions. Or else, if you don't need the data on your SD card anymore, you can replace it with a new one.