An unfortunate lapse in her reeducation after Nomad's memory wipe.
Were TUC set during the TOS Era it'd be easier to believe Uhura doesn't know Klingon since the Federation and the Empire hadn't had that much continuous contact in the century since the Augment Virus struck but by 2293 she should know at least enough Klingon to not have to use a book as she translates.