Inventor fusion with the boss / base feature can not mirror.
However, the same process hole / cut feature mirror can.
Boss / base features how the mirror will do it.
Teach someone how to do that with video, will you?
Thank you for bringing this to our attention. Currently, it is not possible, as you observed, to mirror the first feature in a body. This is a feature that we plan to add to a future relase of Inventor Fusion.