If an attack would KO a Pokemon, can you use Sitrus Berry to heal it first?
No. KO's have to be resolved before the end of the turn, and Sitrus Berry heals at the end of the turn after KO's have been evaluated.
Source: TPCi Rules Team (2020-02-06)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH1_182
SWSH1_182