In theory it is possible to submit a work in progress cut of a film for festival consideration, but this is not recommended unless the film is from a well-known director.
A-list festivals like Cannes are already massively oversubscribed in terms of the ratio of submissions to festival berths. So in reality, incomplete films need to be exceptional to get a look-in, even before they are finished. Even ultimately fantastic films may not show their greatness until completed.
The other key consideration is whether the film will be truly ready for Cannes. If the festival does show an interest in the film, it's berth will be conditional on it being completed in time for the first screening. Cannes history is resplendent with examples of films which were selected, but ultimately met with poor reception and bad reviews because they were rushed simply to be in the festival. Southland Tales, Richard Kelly's follow-up to Donny Darko is one of many films which springs to mind.