What you need is:

DisplayData20YrNoScientific = Map[Tooltip[#, NumberForm[N[#], ExponentFunction -> (Null &)]] &, DisplayData20Yr] 

So check that the tooltip works ok on the data:

Then plot it:

ListLinePlot[DisplayData20YrNoScientific, Joined -> True, PlotMarkers -> {Automatic, 12}] 

What you need is:

DisplayData20YrNoScientific = Map[Tooltip[#, NumberForm[N[#], ExponentFunction -> (Null &)]] &, DisplayData20Yr] 

So check that the tooltip works ok on the data:

Then plot it:

ListLinePlot[DisplayData20YrNoScientific, Joined -> True, PlotMarkers -> {Automatic, 12}] 

Edit

Forgot to mention that your original DisplayData20YrNoScientific

DisplayData20YrNoScientific = Map[NumberForm[N[#], ExponentFunction -> (Null &)] &, DisplayData20Yr]; 

creates a list of strings so there is nothing for ListLinePlot to plot which is why it fails.