Noob Question: Unable to Weight Paint my Mesh So, I made an armature for my bipedal model and successfully rigged it in Blender 2.7. Since then, I updated to 2.9, and then added some Mouth bones to my armature. Now I want to weight paint my model to the Mouth bones, but I'm unable to select bones in the armature. I cannot click it!

Link to my .blend file:
https://drive.google.com/file/d/1XkwYgJpSaftngevlgV5yr7aEJITty4LG/view?usp=sharing
Thank you for any answers and comments. :)
You can select the corresponding vertex groups in the mesh properties, or go to object mode, select the rig, shift select the mesh, enter weight paint mode and select the bone with Ctrl left click.