No other selected objects have wire or boundary edges to use for projection

Why do I get that error message? In the object mode, I first click the smaller object and then shift+click the big object below. Then I pressed tab, then F3, then typed knife project and selected it.

not sure of what the F3 KEY does but you can only use knife on the active object.

Other selected objects are not affected by knife tool.

Happy blending !

Check and make sure that you didn’t add that while in edit mode…They must be separate objects for it to function…
I run into this myself when I forget and not separate out the cutter while in edit mode…

In my case, F3 only brings up the add objects menu…
I select cutter > shift select object to cut > Tab > Mesh >Knife Project

Not true…He is using Knife Project… using one object to cut another…

It seems that this is a bug/feature.

https://developer.blender.org/T90656

Blender now allows multiple objects to be added to edit mode, which complicates the knife project process.

The correct way is to select the item[s] to be cut, enter edit mode and then [CTRL][LMB] the cookie cutter object from the object tree. Then Mesh->Knife Project.

This is prolly a little late for the OP, but might help people ending up here after googling.