I2C Write Operation on TCA9539
Oh. Simple enough. You begin trasmission to 0x74. Then you end it.

Then you begin transmission to 0x02. That means you really sent a transmission to device address 0x02. Instead of device 0x74 register 0x02.

For that you would do

Begin transmission 0x74.  
Write 0x02. Register
Write value.  
End transmission. 
If back = 0, you successfully
wrote value to register 0x02 on device 0x74


By ending the transmission you send a i2c stop condition, ruining the write or read from the device you began transmission to.