October 2015 - last edited October 2015
Pretty sure it's a bug.
I used to have access to Hard Mode, but the button that switches Normal to Hard is 'Characters' for me and bring me to the character screen.
I've finished all the Normal Squad Cantina battles I can, and when I first go into the Squad Cantina screen I get access to the level 3 Hard mode Squad Cantina battles (loads up by default), but I rather do the Level 1 and 2 ones first...
EDIT: Just saw this...
"* Known Issue: Sometimes the difficulty buttons will appear at the bottom of Cantina Battles. This is unintended and will be fixed in the next client update."
I guess what I'm seeing is the beginning of the next set of battles which I don't have access to yet.