Mirror modifer: Failed to set value?

Hi!
I’m fairly new to Blender and have been looking at sculpting tutorials and have come to learn that when you want to duplicate something without using the copy command you use the Mirror Modifier, like when making the eyeballs. However I’ve come across a problem, when I want to duplicate my uv sphere the Mirror modifier doesn’t work?
Because what happens is that when I try to select my sphere with the eyedropper tool (or the menu) an error message appears saying “Failed to set value.”
I’ve tried googling but I nothing came up to why this happens or how/what I’m doing wrong.

Also when I go into the Mirror Object menu there aren’t any avaliable objects to choose between.

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  1. Always read the tooltip, in case of the ‘Mirror Object’ of the ‘Mirror Modifier’ it says: Object to use AS mirror, not object ‘to’ mirror, so you need to set a different object. If a mirror object is defined, the modifier uses the location and rotation of a different object to define a mirror plane. So, just create an empty at the origin and set that object as the mirror object of your eyeball.

  2. Alternatively, you don’t have to set a mirror object at all, if you offset your eyeball in EDIT MODE. In this case the eyeballs location and rotation is used to define the mirror plane, make sure the object’s transforms are cleared.

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