It's better to speak about it roughly as a concept without getting into the specifics, such as "i morally cannot find someone guilty of X". The judge and lawyers will know what you're getting at and you won't have "tainted" the entire jury pool which could get you in trouble in some courts.

But it's best to say nothing and get on jury duty so you can use your right to try to nullify if circumstances warrant it. You still can't mention it when you've been selected and you still need to participate in jury deliberations but you can ask questions to other jurors and raise doubt in the prosecution' case/ defendant's guilt. You also aren't required to justify your vote but if you need to explain to your fellow jurors you can just simply say "in my heart im just not able to convict"