Epilog with text not shown

Use ImagePadding with a large enough value and set the option PlotRangeClipping to False:

Plot[x^2, {x, -10, 10}, Frame -> True,
 Epilog -> {Text[Style["S", FontFamily -> "Helvetica", Black, Bold, 20], {-12,10}]},
 ImagePadding -> 30, PlotRangeClipping -> False]