It's because the APU is running that you can't change the fuel quantity in the left tank. Turn on external power, turn off the APU (that way your FMC programing doesn't get erased). Also. When you turn external power off, wait until the APU is running (the RUN light remains ON).

If I'm going to sit at the gate for any length of time, I turn on external power and external bleed air. I only turn the APU on just before starting the engines. That way the APU isn't burning fuel.

Hope this helps.

Mark