Okay, according to thgis thread:
and this one:
These libraries used to be included in the Blynk library, but aren’t used anymore, so you can remove or comment out the line that says
#include <ESP8266_SoftSer.h>
Presumably, you’re using a very old sketch that you’ve found from somewhere, so I’m guessing there may be other issues with using it with the current versions of the Blynk library, server and app.
Pete.