It looks like there is an error on the RS485 Breakout Board schematic.
There is no RTS net connecting JP9-4 to U2. Also, shouldn’t the load resistor R4 be 120 ohms?
See attached pic.
Apparently this is a known issue https://github.com/sparkfun/RS-485_Breakout/issues/1 - feel free to ping/bump the issue filed @ that github link