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.
Mac not turning on after factory reset without installing macos
