I have run into this before, it seems reference lines won't work with automatically binned fields. I believe they are read as Tableau differently than other continuous, numeric fields.
If we look at the Sales pill, we see it's a measure:
If we look at the Bin pill - we see it is actually a dimension, and Measure is grayed out:
If you create your own numeric bin, make it a dimension, make it continuous...
then you can use a reference line:
Hope this helps,