Error using SD card module: no matching function for call to
The function is defined as

// Methods to determine if the requested file path exists.
boolean exists(char *filepath);


Source: https://github.com/adafruit/SD/blob/master/SD.h#L90

You call it with a File object, not the filename. You probably mixed up the variable names:

File myTest;  

void setup()
{
  //
  char myTestName[] = "MyTest.txt";  

  //[...]
  if (! SD.exists(myTest)){
     myTest = SD.open(myTestName, FILE_WRITE); 
     myTest.println("My 1st Line of Data");
     myTest.flush(); 
  }
//.. 


Just exchange myTest for myTestName in the SD.exists() call.