Install brew in MacOS 10.13.6

Steps

Download following packages, install first two packages.

Xcode_10.1.xip
Command_Line_Tools_macOS_10.13_for_Xcode_10.1.dmg
Kernel_Debug_Kit_10.13.6_build_17G10017.dmg
Swift_5_Runtime_Support_for_Command_Line_Tools.dmg
Swift_Playgrounds_Author_Template_for_Xcode_10.xip
Font_Tools_for_Xcode_11.dmg
Additional_Tools_for_Xcode_10.1.dmg

Run following command to install

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

References

All Downloads
Install Homebrew · Mac M1