Cannot determine address of connected I2C device (simple I2C scanner)
According to the schematic of the PCB and the datasheet of PCF8574 the I2C address can be calculated after figure 10:


I'm not sure which version of the IC you are using. The schematic says PCF8574 and you mentioned the PCF8574A.
However if it's the PCF8574 the address is 0x20, if it's PCF8574A the address is 0x70.