Hi,
It looks like what you have is two bodies intersecting and being displayed at the
same time. There are two bodies but they are both turned visible and fusion will
render this as best it can.

It is difficult to know what it is you are trying to achieve, but if what you want is
the cylinder to cut out a piece of the sphere and become a replacement core then
there are several different ways. One is like this, you need to first use the core
body to do a Combine Cut.

Run the command and your Target is the Sphere and your Tool is the core.
Make sure you Keep the Tools.


What this does is cut a hole in the sphere with the core as a Tool.

And since you Kept the Tool you still have the core.

And you get what you wanted.

It seems that you are new to the Forum so I would strongly suggest looking
at RULE #0 to learn Fusion better. Thanks for the question, images are good
but next time also attach your file so we can see what you have done.
Cheers
Andrew