So I am sculpting a head, while following a tutorial https://www.youtube.com/watch?v=QmmSAsn_mOk. I created eyelids and used the mirror-modifier on them (also subsurf and solidify). Now it´s time to merge the eyelids with the head and sculpt on them to make them seamlessly connect to the head. The problem are the lower eyelids: the modifiers have Applying disabled; when I click on apply it says modifiers cannot be applied to multi user data. I applied the upper lids just fine, not knowing how they differ from the lower ones. I never came across multi user data before neither at which point it was tackled; how can I fix the lower eyelids in order to make their modifiers applyable? Here´s my file https://www.dropbox.com/s/iyg6tn0s4uclycm/head8.blend?dl=0
You have a linked duplication of your eyelid object that is called Sphere and that is currently hidden. You can see in the Object Data panel that the shared mesh is called Sphere.002. You can't apply a modifier to an object that shares the same mesh as another object. You either need to make the linked duplication single (press on the 2 button), or Object > Relations > Make Single User > Object & Data, or remove this object:
According to Kuboa, a new patch was accepted and added to Blender about this workflow. Now it will warn you that applying the modifier will make the object-data single-user, but it does apply it if you're OK with that. Available right now in 3.2 Alpha.
Select the lower eyelid and go to the Object Data Properties tab (green triangle icon).
Next to the name of the mesh (Sphere.002), you can see the number 2 : this means the same mesh is used by another object (maybe you used Alt+D to duplicate it, or something like that.)
Just click the number to make it "single-user", that will fix the problem.
For your information, the other object using the same mesh is "Sphere", which is hidden in your scene.