I think you don't have the exfat drivers to write. You could try the
-ro option in your mount command, it should work (but you obviously won't be able to write on your card^^).
Here's what I found on Google (in French) : ubuntu-fr.org
Exfat (or fat64) is the latest AND non open-source file format by Microsoft, to overcome the 4gb limit size of fat32. To access exfat with read-write, add the ppa
relan/exfat and install the package
sudo add-apt-repository -y ppa:relan/exfat
sudo apt-get update -qq
sudo apt-get install exfat-utils
Another solution would be to format your card in NTFS (non open-source, but Ubuntu should be able to write it with
ntfs-3g package installed) or EXT4 format (free open-source yeay!), but it depends on the use you have of the card.
Here you can learn more about the various formats.
To change the format of your card, you can use a GUI programm like Gparted (root access needed) :
sudo apt-get install gparted