So the title says it all basically : I tried factory resetting my 2018 macbook using the apple support guide but I failed to install the latest version of macos as the last step and now I can't even turn on the mac to get to recovery mode and install it. Can someone help me please ?
You didn't say what OS the Mac was using.
Without an operating system there is no computer to turn on. All you have is a case full of electronics that do nothing.
If this is the procedure you followed, you need to use the bootable flash to install the OS: Create a bootable installer for macOS - Apple Support
If by "reset" you mean you erased the drive, you're going to have to find a working Mac to create the bootable Flash drive.
If you didn't erase the drive: Reinstall macOS - Apple Support. Choose the OS as appropriate.
You didn't say what OS the Mac was using.
Without an operating system there is no computer to turn on. All you have is a case full of electronics that do nothing.
If this is the procedure you followed, you need to use the bootable flash to install the OS: Create a bootable installer for macOS - Apple Support
If by "reset" you mean you erased the drive, you're going to have to find a working Mac to create the bootable Flash drive.
If you didn't erase the drive: Reinstall macOS - Apple Support. Choose the OS as appropriate.
Mac not turning on after factory reset without installing macos
