Invalid operands of types 'const char [21]' and "float" to binary 'operator+'

[blh64]

You can't add (+) a float variable 'voltage' to a constant string "The Conductivity is" in C/C++.

The usual way to do this is to break it into 2 print satements

Serial.print("The Conductivity is ");
Serial.println(voltage,2); // 2 decimal places