Add a Target Line Straight Across Excel Chart

My first suggestion would be to use one of the techniques discussed here: https://peltiertech.com/Excel/Charts/AddLine.html

I obviously don't know what you tried, but I had no trouble adding the horizontal line by either method. When the horizontal line series is converted to an XY scatter chart type, then it mostly ignores what the other column data series are doing. How carefully did you follow Peltier's instructions? What did you do differently?

As a gross oversimplification of Peltier's instructions, I performed the following steps in your file:

1) Select the chart -> Bring up the Select data dialog -> Add a data series -> For the Values field, click on B1 -> click OK until the Select data dialog is dismissed.
2) Select the single column of the new data series -> Change chart type -> one of the XY scatter chart types
3) Select the new XY scatter series -> Layout -> add error bars
4) Select the vertical error bars and delete them
5) Select the horizontal error bars and format them as desired.
6) If desired, select the new XY scatter series and format to have no marker.

I'm not sure what you tried before, but that should work. If it doesn't, let us know where it goes wrong and which step you have trouble with.

It doesn't matter how your existing data is organized, you just need to add two data points and create another line, making it a combination chart.