This volume can not be resized
As per this Apple support question:


  You cannot dynamically resize an ExFAT partition, like you can an HFS+ partition on a GPT volume.  It's a limitation of the ExFAT file system, I believe.  You'll have to destructively repartition and reformat.


So, you have to reformat the entire drive. Remember THIS WILL DELETE ALL OF THE DATA ON THAT DRIVE, so make sure you move all of your files off before you do this. Let me know if this answer wasn't clear or you want help walking through the steps to do this.