Why is Proto-Indo-European form for light *lewktom when Russian has satem лысина (lɨsʲɪnə) bald spot?
The root *l-wk- has dozens of descendants all across PIE, including Russian luč. Forms like that, Sanskrit roka, etc show clear evidence of a PIE velar *k.
I'm not an expert on Slavic, but I'd assume lysina has
/s/ not from the satem shift but from some later development, possibly affected by the front vowel after it. (Alternately, it might come from a different root altogether, or be a borrowing, etc.)