how i put string value from BH1750 -light sensor to if else condition?

Other Hardware Science and Measurement
2 
if (digitalRead((String)lux) <= (1200))

DigitalRead does not take a String as a parameter. It takes a pin number (int). Is there really a lux pin to read? DigitalRead returns a HIGH (1) or a LOW (0). So always less than 1200. Can you explain what that is supposed to do?

Maybe what you want is:

If(lux <= 1200)

else (digitalRead((String)lux) > (120));

The else structure takes no arguments.