What does Sacred Bell do?

It was seemingly added in an errata / playtest pack. It makes Bronzong resist Dark and Ghost-Type moves one step further

It can at least be found in the February 2016 errata / playtest packet which is inside the zip file found under "Download PTU 1.05 here" from the official website. The errata states:

[...] Ability: Sacred Bell

Static

Effect: The user resists Dark and Ghost-Type Damage one step further. [...]

Notably, Bronzong is Steel/Psychic so it would ordinarily be hit super effectively by Dark and Ghost-Type Moves; this ability causes them to hit with neutral effectiveness.

This was the earliest mention of the Ability I found, though perhaps an earlier version of it exists somewhere also. Note that this errata is a playtest pack; to quote its opening:

This is a supplementary playtest document for the fanmade Pokémon Tabletop United roleplaying system, written for use with the 1.05 release version of PTU.

We’ll occasionally release packets like this one to test experimental content and give us a sandbox to play with ideas that aren’t quite ready for a full release but which we want to get feedback and comments on. Everything you see here is subject to change!

This included quite a few changes besides just the apparent addition of the Sacred Bell Ability, such as completely altering how the Flinched Affliction works.