Hi there, I get the information from your screenshot that your SD card is unmounted. In order to format it to the exFAT file system, you need to manually mount it in Disk Utility on your Mac device first.
You can find that your SD card is not mounted on your computer as long as you see the bottom menu bar in Disk Utility. Just click the Mount button on the top menu bar then your SD card can be mounted without any trouble.
As to the "The disk you inserted was not readable by this computer" notification, you can try the following methods to get it fixed:
Fix 1: Do a basic troubleshooting
The methods provided below are able to perform basic troubleshooting. Do not hesitate and try them right now!
- Check if the disk is encrypted
- Check the disk on another computer
- Change the USB cable and USB port
- Reset USB port
Fix 2: Re-insert the external hard drive
Fix 3: Update the drivers on your Mac
Fix 4: Run First Aid to fix the unreadable disk
If you fix your SD card well, then you can format the SD card to exFAT on your Mac device in one go.