May have figured it out. I'm relatively new to this program too, and did it without trying so bare with me. I opening another drawing to try to duplicate what I did and, go figure, it is behaving and I now I can't get it to come in as "unknown units".
I had to unlock the layer that the scale bar was coming in on(it came in locked, I did not lock it). I had a drawing that had a scale bar already in it. This was displaying properly with correct scale notation. When I changed the vp scale and tried to put a new scale bar in to show the new scale, it came in with the "unknown units" message. After I posted on your original post i was messing around trying to insert a scale bar as a block, through the Insert tab. I clicked INSERT tab and selected the dropdown under INSERT on the BLOCK panel. I selected MORE OPTIONS and in the NAME dropdown selected a random scale bar from those visible the EXPLODE box was checked so I left it like that. I brought two in, one exploded and one not. The exploded one had text where the numbers should be while the unexploded one came in with "unknown units". Both were way bigger than I wanted but my hope was that I could edit to do what I wanted them to. When I tried to delete the EXPLODED one a message came up that said they were on a locked layer.
This is where I'm not sure what I did that worked. I selected the block scale bar that was brought in exploded and went to home tab. The layer that was automatically generated was locked, I did not lock it that was how it was generated. I can't recall if both scale bars I brought in as blocks were on the same layer or not but when I unlocked that layer I was able to delete the scale bars brought in as blocks.
When I jumped into and out of model space in the layout the dropdown at the top in "layout elements" panel was visible(this is how I originally attempted to bring the scale bar in). I selected the scale bar I hade before, which previously came in with "unknown units" and this time it displayed the scale as it should. Only thing I can figure is that it had something to do with the layer being locked.
Worth a shot, it worked for me.
Mike