Blender: Active Object is Not a Selected Mesh – Ctrl+J Doesn’t Work

Blender: Active Object is Not a Selected Mesh – Ctrl+J Doesn’t Work

While working on a new project in Blender 3.2.0, I ran into the following problem: I wanted to use Ctrl+J to join several meshes/objects, but it didn't work. Blender said "Active object is not a selected mesh". I started looking into what the problem was, and I came across the hint…