In the 2.92 manuel, in
Modeling/Sculpting & Painting/Modes/Weight Paint/Using Vertex Groups/Vertex Groups for Bones
It is said:
Select the armature in 3D Viewport, and bring the >armature to Pose Mode with Ctrl-Tab, or the 3D >Viewport header mode selector.
Select a desired bone in the armature.
Select your mesh with LMB and change immediately >to Weight Paint Mode. The >mesh will be colored according to the weight (degree) that the selected bone >movement affects the mesh. Initially, it will be all blue (no effect).
I think in 2.92 Blender version that's impossible.
Instead you have to select the armature in object mode and add the mesh object to the selection, and change immediately to Weight Paint Mode.