This worked for me. For future reference in case anyone else is using this with Repetier-Server, the pause.g action did not execute even after the change to my M591 command, but it did send a different notification that I was able to catch via Repetier's G-codes response to event and send the pause command from there. I used this regex in Repetier.

^Filament\serror\son\sextruder\s\d:.*

I am attempting to utilize the filament runout sensor when running a job via Repetier-Server over serial. What I am looking to do is just send a host action command to the server when the filament runs out. Here is the relevant line of my config.g

M591 D{var.Tool_Num0} P{var.E0_filSensTyp} C{var.E0_filSensPin} L{var.E0_filSensCal} A1 S1				;Configure Filament sensor for tool (D#) Sensor (P#) on pin (C"#") and calibration (L#) enabled on any extrusion move (A#) and enabled at startup (S1)

And here is my pause.g file, which just sends a host action command to Repetier-Server. I've tested the command independently by just entering it in the Duet web interface console and this does give the intended result (pausing the print) on the Repetier-Server side.

;pause.g
M118 S"//action:pause" ;Pause job on the Repetier-Server

The pause action triggers when running a job though the SD card, but not when running a job over serial via Repetier-Server. Is there a way to enable the filament runout sensor when running a job over serial?