Why is the generate rig button greyed out?

Question

I have appended a Rigify metarig from the Armature category from an older blend file via the File > Append... main menu. In the older project, the metarig's (Re-)Generate Rig button is not grayed out and works. But for the appended metarig, the button is greyed out and you can't (re-)create the animation rig.

What is the reason for this? How to append it correctly?

Answer 1

Do not append metarigs from the Armature category. Instead, append them from the Object category.