If an oscilloscope shows 0 (no signal/low signal), the signal source is not connected or not sending anything; what is the signal source?
That would not be a problem related to your program.
As indicated, your code does not compile. Because you did not close loop() with a }, void writeReg1 (byte Add, byte mask){ is inside loop() where it should not be. Add a } after the delay(5000).
I'm not familiar with the ADS7066 so can't advise further.