How do I activate object eraser on my iPhone 14 Plus?

That feature is available just on iPhones running iOS 18.1 or later versions AND compatible with Apple Intelligence (iPhone 15Pro or 15ProMax, or all models belonging to the 16 series).

Regards

Giulio

I believe you are referring to the Clean Up tool which is apart of Apple Intelligence. That is only supported on the iPhone 15 Pro and later, since older models lack the hardware and RAM to run it. So you won’t be able to use it.