If my opponent has no Weakness and I use Unown VSTAR's "Star Cipher" Ability, will my opponent's Pokemon become weak to Psychic?
If an opponent's Pokemon has no Weakness printed on the card, Star Cipher will give that Pokemon a x2 Weakness to Psychic.
Source: Silver Tempest FAQ; TPCi Rules Team (2022-11-10)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH12_66
SWSH12_66