katakana letter o missing

What makes you sure it's missing? I reviewed it multiple times, it's not missing for me.

Oh right. I'm almost sure this is intentional, because wo katakana is often not taught at all in other beginner materials as well, because it's an extremely rare character in the wild. While hiragana wo is used for object particle, there's no such usage for katakana. When there's an intention to literally say "wo" as a part of a katakana word, a conjunction of other characters is often used instead, such as "wa" + "o".

This is commonly requested, so I'll add it either in the next version or the one following.