'12 Years a Slave' premiered at the 18th annual Capri, Hollywood Film Festival that kicked off in the country on December 27 without any of its big stars in attendance.
Nyong'o was initially expected to attend the event but she 'could no longer go' due to other commitments, reported Ace Showbiz.
"Due to other commitments that prevented her from going, Nyong'o's reps let the Capri festival know over two weeks ago that she could no longer go," a spokeswoman for Lionsgate, whose subsidiary Summit is the film's sales agent, said.
Summit has since cleared up the issue, saying that the posters were unauthorised and had been withdrawn from circulation.
Italy's BIM Distribuzione also apologised for the mishap.