Hi @Salwa_Razak,
If you cannot find the 4.27 source then follow the post which explains how to register for the source.
Linking to GitHub account is successful but does not grant access - AnswerHub - Unreal Engine Forums
Then clone the 4.27 source code
$ mkdir source\repos
$ cd source\repos
$ git clone https://github.com/EpicGames/UnrealEngine.git -b 4.27
$ cd 4.27
$ setup.bat
$ GenerateProjectFiles.bat -2022 or 2019 if you want
open Ue4.sln using Visual Studio
Right-click UE4 project and “Build”
Then carry out the instructions above for UnrealLightMass project