I’ve had this issue a few times before, and I can’t seem to get a fix that’ll last long enough, but there are instances where I am unable to select the bone and the object together for weight painting. It’ll select one of them, but it outright doesn’t work when I try to select both of them to parent together. Is there a reason why the 2.9 series of Blender does this, because I had many of these same issues in 2.90.
Before anyone asks, no, I don’t have “Lock Object Modes” checked. That was the cause of my woes when I first used 2.8, but I found that out and unchecked the box. This appears to be something unrelated to that.