56 and 57 are wired together, so there isn't technically an issue with this.
In order to actually debug what's going on, you would need to perform the voltage checks on page 17 and let us know what is correct and what isn't correct.
If you have one LED on one of the 6922 sockets that isn't lighting up, but the other three are lighting up, then you almost certainly have an issue with the underlying build that needs to be addressed first. I fix a LOT of Reductions with the repair service and I think I have maybe seen one with actual mistakes made, whereas the rest were just poor solder joints that didn't capture all of the connections on a given terminal. This is extremely common on the ground buss wires that run from the input RCA jacks around the 9 pin sockets and into the terminal strips. Pretty much every Reduction I have repaired has a loose wire on pin 9 of at least one of the 9 pin sockets, and this is definitely something that will cause one LED to not light up.