If a champion uses their Lay on Hands ability against an unwilling undead creature, does s/he need to make a touch attack roll or does it automatically cause damage?
Lay on Hands does not require an Attack Roll
The Lay on Hands focus spell has a range of Touch, but does not have the Attack trait. Touch Range states (CRB p. 304)
You can usually touch the target automatically
Because the spell does not have the Attack trait, it does not specifically require an attack roll, and touch range spells without any additional requirements hit automatically. Lay on Hands does however have a basic Fortitude save when used against undead.