Unfortunately, Greek verbs don't behave as predictably as the learner might want... While the aorist often can be derived from the present stem by adding a sigma, this isn't always the case. There are some verbs which use a different stem, and verbs whose stem ends in a liquid (λ,μ,ν,ρ) lose the sigma.
There was a discussion not too long ago about this here, you might find it has some additional information which is helpful.
IPHIGENIE: Kann uns zum Vaterland die Fremde werden?
ARKAS: Und dir ist fremd das Vaterland geworden.
IPHIGENIE: Das ist's, warum mein blutend Herz nicht heilt.
(Goethe, Iphigenie auf Tauris)
The verb leukainw is a liquid verb, the stem ends with a -n-. When a -n- is followed by a -s-, there are two possibilities.
1.- The -n- assimilates to the following -s-. It's very frequent and the -n- "disappears"; pants > pans > pas "all". 2.- The -s- assimilates to the preceding -n-. Less frequent. This is the case in these verbs; leukansai > leukanai.
It's a simple assimilation characteristic of this kind of verbs.