Humans don’t ‘know’ everything – and that is true for even the person with the most inclusive epistemic position.
In the context of a discussion about ethics and the goals which necessarily underlie them, it follows immediately from this that all goals will be ‘known’ in other than purely ‘objective’ ways.
For some this leads to teleological agnosticism (we cannot ‘know’ goals), which pretty much necessarily entails ethical agnosticism (we cannot ‘know’ what is right/wrong).
If, however, we take ‘subjective’ things such as feelings, desires, intuition, tradition (and especially if we mix these with logic/reason) seriously enough to be epistemic sources (at least possible valid sources of ‘knowledge’), then we can be perfectly justified in saying that we ‘know’ something is right/wrong (note the quote-marks around ‘know’).
And if this is not objective and flawless enough – then we might ask, ‘who ever said we were omniscient?’