The problem most often encountered when people argue for objective morality is that they fail to understand the implications of what it means for a property to be objective. This is true both for those claiming morality is a property that can be perceived and those claiming to possess an objective standard of morality.
Objective truths are those true independently of the perspective of thinking creatures like ourselves. When I say the mass of a tree is objective I mean that the mass won’t change merely because of a change in opinions or that even if there were no humans (or other sentient creatures) the tree would still have the same property of mass. Conversely when I claim a trait is subjective I mean it is dependent on opinions and because of this it wouldn’t exist if there were no sentient beings around to give opinions. The beauty of a tree would be one such subjective property as beauty is a judgment not a property that can be detected independently of observers. Likewise an objective property for a given object is one that could be detected independently of that object’s other properties.
For example, humans find certain patterns of audio waves pleasing. While humans do perceive differences in audio waves we can, and have, created devices for measuring audio waves, and they are far more accurate than we are. These devices don’t depend on imitating our perceptions of hearing but rather measure the amplitude and amount of waves directly. However with the “goodness” of music the only way to devise a machine to detect it would be to make the machine mimic our subjective judgments. Such a machine would only be accurate insofar as it had the ability to mimic human perception and there would be no way to make it more precise than our human inputs. Also such a machine would be incapable of objectively determining whether La Campanella or Beethoven’s Symphony No. 5 was really the better musical composition because it would rely entirely on subjective value judgments in order to function.
This is but one example of the difference between detecting objective and subjective properties. The same distinction could be made between the chemical composition of food and individual gastronomic taste or between the height of a structure and the perception of tallness. Those claiming that morality is property that can be perceived are saying it is possible, at least in principle, to build a machine to objectively determine what balance of agency and outcomes is “correct” which doesn’t just mimic human inputs. The problem is concerns over both agency and outcomes are entirely subjectively driven. You won’t find the “badness” of an outcome in the outcome itself, say going bankrupt, but in human preferences, like our desire to be financially stable. Without some kind of subjective preference no outcome, or amount of agency, would be “better” than any other.
Moreover even if we accept some purportedly objective moral standard, while there would be objective ways to move within that framework there still wouldn’t be an objective basis on which to decide which direction we ought to move. Worse yet there wouldn’t be a reason independent of subjective preference to decide which among many competing standards we should adopt. This doesn’t mean our choices will be arbitrary, no doubt the choices of human morality are heavily dependent on the type of creatures we are, but it does mean that they are subjective choices even if they are subjective choices with outcomes preferred by everyone. The basic failure when arguing for objective morality, whether secular or religious, is always of noting subjectively preferred outcomes, mostly pleasure over pain, and then incorrectly extrapolating that this preference somehow is a part of reality independent of our preference, it is not. The sooner we acknowledge this the sooner we can have a more thoughtful conversation about morality.