Arduino Nano Every

I have a Nano Every and I can’t find a library to manage Low Power.
Could you tell me how to put it in sleep mode?

use the avr/sleep.h library - there's really no benefit to the arduino wrappers around it. it's part of avr-libc, docs are avr-libc: <avr/sleep.h>: Power Management and Sleep Modes

works for all avr's - both megaavr's like the every and classic avrs like uno/nano/etc

Assuming this is indeed the nano every, not one of the other two "nanos" that recently came out, both of which are based on different processors, and have basically nothing in common with eachother or the every or original nano other than the physical shape of the board.