Problem with building ue4.18

i had this message when i try to build UE4

Severity Code Description Project File Line Suppression State
Warning The referenced project ‘UnrealBuildTool’ is targeting a higher framework version (4.6.1) than this project’s current target framework version (4.5). This may lead to build failures if types from assemblies outside this project’s target framework are used by any project in the dependency chain. AutomationUtils.Automation

any idea ?

You have to check the target framework for your project and for the different solutions. Very likely there is a mismatch.

Here is a reference: How to: Target a Version of the .NET Framework - Visual Studio 2015 | Microsoft Docs

Which VS version are you using?

am using VS 2017