I ran into the same problem today on a 2016 Nissan Leaf with a fully up to date MX808.
The MX808 leads you to believe that it will pair new keys and walks you through the procedure, but you end up with the error: "The operation is incomplete try again after confirming the operation conditions" I tried this a bunch of times, but it did not work, and it left me with no working keys...so I guess you could say that it kept part of it's promise in that it disabled all the existing keys, but it will not program new ones.
I called Autel support and they stated that the MX808 will not ever work program/pair any Nissan push to start keys, such as the key that comes with the Leaf.
When I asked why the MX808 let's you get as far as deleting the existing key parings without letting you pair new keys, and what they could do about fixing this serious problem, their response of "not my responsibility" had an air of indifference with an undertone of indignance.