Do you have to use a skill from the KRB when a character doesn’t have it or should you use a generic characteristic?

Let’s say a player wants to use a sword to fight an enemy in Call of Cthulhu 7e but didn’t pick that specific skill for the character, should you as a keeper make the player roll a sword skill (20%) from the Keeper Rulebook (p. 64) against a difficulty of the base 20% (with penalty?) or should the player make a general DEX role with penalty die?

Basically, should you pick a skill on character creation to be able to use it or can you use any skill with base difficulty if your character doesn’t have it?