The bug

Craft a trimmed armor at a Smithing Table

This sentence has a grammar problem. The word "armor" is uncountable, just like the word "water". There is no "a", "one" or "two" waters. And similarly, no "an armor" either.

Instead, it should be either "trimmed armor" without the "a", or "a trimmed armor piece". And considering this is about one single piece of the full set specifically, the latter is the correct one.

To reproduce

/tellraw @p {"translate": "advancements.adventure.trim_with_any_armor_pattern.description"}