This is a bit of a misunderstanding. WaniKani asks you to answer with ぎゅう for the kanji item because they taught you ぎゅう in the lesson. But ぎゅう and うし are both valid readings of the kanji as a concept.
When it’s a vocab word by itself, it is うし.
This is a bit of complicating factor, actually, because signs tend to use abbreviations and different grammar from sentences, so it’s possible to imagine some scenarios where 牛 by itself is うし and scenarios where it’s ぎゅう on a sign. For instance, 牛 (ぎゅう) could be used as an abbreviation of 牛肉 (ぎゅうにく) and in that case, you would probably read the kanji as ぎゅう.
But if you’re just reading a normal text with 牛 in a sentence, it will likely be うし if it’s by itself as a word.