Always check the battery first.
ARM3: AA battery positive (+) should go into the wrench first. Battery negative (-) should contact with closing cap.
ARM6: make sure the AAA batteries are placed to battery holder/cartridge with correct direction (+ and -), put cartridge to wrench correctly. if wrench can't power up with new batteries and after checking position, it could be battery holder defective. https://www.durofix.com/product-support/2021/9724/
Also possible PC board become defective (see ARM601 can't turned off)