Hey -
When you built the engine from source did you configure the directory as an Unreal Engine installation? To do this you can go into the folder where the source code is and then go into Engine->Binaries->Win64 and scroll to the bottom of the folder to “UnrealVersionSelector.” Here is a link that walks through the entire process of getting source code from Github with the necessary steps near the bottom of the page.
Once it has been done the other thing to check would be to right click the .uproject file for the project and select the “Switch Unreal Engine version” and choose the engine version you’d like to work in.
Cheers