The Nun is a genuinely great movie which is kinda creepy. If you want to watch The Nun online (the full movie) then here are a few options (we checked these as recently as June 2019):
Amazon Prime Video
The entire film is available on Amazon Video for free, and you can sign up for Amazon Prime Video completely free in 10 seconds and have access to it immediately, plus a whole bunch of bonus content, 100% free!
Just cancel before the 7 days is up and you don’t pay anything ever, simple. If you’re wanting to watch films like The Nun online for free and in high quality right now, then a free trial with Amazon Prime Video is your best option.
Where else can you watch The Nun online?
Hulu have The Nun on their platform, and you can watch it with a trial too - however, it's a bit harder to cancel the trial through Hulu, and that's why I'd always recommend to watch The Nun through Hulu.