Incapable of Creating a Manual Line (Threshold) on a Box and Whisker Chart

To do this natively in Excel with that chart type you could plot the entire data set then modify the thresholds to show as mean lines:

The results:

The downside being the threshold lines do not run the width of the chart.

You could use Python in Excel to create a Box chart with the Seaborn library.

This is what I made with your data:

If you want to explore the Python route, I recommend loading the Anaconda Toolbox office addin. It helps a lot in creating these charts. It will give you a menu to work with:

I tabled your data and built the initial box chart. I then made revisions to match the desired result.