I can't understand

I Have the maximum voltage at the power supply 20 volts. Where there will take 21 volts?

But my voltage only 20V, it never be 21V.

If i set M911 S16.0:21.0:22.0 and my voltage 20, then auto-pause and auto-resume will be off, and only save ressurect file will be active and start when voltage drops to 16V.

Do I understand correctly?

Firmware Name: RepRapFirmware for Duet
Firmware Electronics: Duet 0.85
Firmware Version: 1.19+5 (2017-08-30)
Web Interface Version: 1.19

I try this function and cant find file sys/resurrect.g on SD card.

i Seach here:

https://yadi.sk/i/QTIQ2zrk3MWJKi

I make all as wrote here: https://duet3d.com/wiki/Setting_up_to_resume_a_print_after_a_power_failure

in config.g

;Setting up to resume a print after a power failure
; For 20V power
M911 S16.0:17.5:19.5 // auto-pause at 17,5V, then either stop at 16V or auto-resume at 19.5V

powerfail.g
G91
G1 Z2 F2400
M83
G1 E-2 F3600

powerrestore.g
M83
G1 E2.5 F600

resurrect-prologue.g
M116
G28
M83
G1 E2.5 F600

What can I do to make the function work?