So M365 applications use a permissions system that allows people in your tenant (your company) to automatically see, only people with a link, specific email access, etc. are you sure it’s actually publicly available? The error message you’re describing seems to be saying otherwise.

Last, attempts to incorrectly obtain content that’s not been shared with you does get logged automatically and quite visibly in MCAS (ms security suite thing) so if you’re worried about that… just so ya know.