WixUIExtension.dll could not be loaded

I'm trying to make a wix installer. for a web application.

the following is my wsx v3.11 File

<?xml version="1.0" encoding="UTF-8"?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
  <Product Id="Guid" Name="TestInstaller" Language="1033" Version="1.0.0.0" Manufacturer="CompanyName" UpgradeCode="Guid1">
    <Package InstallerVersion="200" Compressed="yes" InstallScope="perMachine" Platform="x64" />

    <MajorUpgrade DowngradeErrorMessage="A newer version of [ProductName] is already installed." />
    <MediaTemplate EmbedCab="yes" />
    <PropertyRef Id="WIX_IS_NETFRAMEWORK_462_OR_LATER_INSTALLED"/>
    <Condition Message='This setup requires the .NET Framework 4.7 client profile installed.'>
      <![CDATA[Installed OR WIX_IS_NETFRAMEWORK_462_OR_LATER_INSTALLED]]>
    </Condition>
    <Feature Id="Complete" Title="TestInstaller" Description="TestInstaller" Level="1" ConfigurableDirectory='INSTALLFOLDER'>
      <ComponentGroupRef Id="ProductComponents" />
      <ComponentGroupRef Id="ProductBinComponents" />

    </Feature>


    <UIRef Id="WixUI_Mondo" />
    <UIRef Id="WixUI_ErrorProgressText" />
  </Product>

  <Fragment>
    <Directory Id="TARGETDIR" Name="SourceDir">
     <Directory Id="INSTALLFOLDER" Name="Test Installer" >
        <Directory Id="INSTALLBINFOLDER" Name="bin">
        </Directory>
      </Directory>
    </Directory>
  </Fragment>

  <Fragment>
    <ComponentGroup Id="ProductComponents" Directory="INSTALLFOLDER">
      <Component Id="ProductComponent" Win64="yes" Guid="*">
        <File Source="C:\Temp\Publish\Web.config" />
        <File Source="C:\Temp\Publish\NLog.config"/>
        <File Source="C:\Temp\Publish\Global.asax"/>
      </Component>
    </ComponentGroup>
    <ComponentGroup Id="ProductBinComponents" Directory="INSTALLBINFOLDER">
      <Component Id="ProductBinComponent" Win64="yes" Guid="*">
        <File Source="C:\Temp\Publish\bin\Antlr3.Runtime.dll"/>
        <File Source="C:\Temp\Publish\bin\Antlr3.Runtime.pdb"/>
      </Component>
    </ComponentGroup>
  </Fragment>
</Wix>

my problem here is that I don't know what this error message means and by extension have no clue how to fix it.

Either 'Microsoft.Tools.WindowsInstallerXml.AssemblyDefaultWixExtensionAttribute' was not defined in the assembly or the type defined in extension '..........\Program Files (x86)\WiX Toolset v3.11\bin\WixUIExtension.dll' could not be loaded.

1. RTF License File: First make sure you have created your own RTF license file (using WordPad or similar) and then specify to use this RTF file in your WiX source like this:

<!-- Shown for context (one of several possible dialog sets): -->
<UIRef Id="WixUI_Mondo" /> 

<!-- The crucial variable that must be defined (for this dialog set): -->
<WixVariable Id="WixUILicenseRtf" Value="TestLicenseAgreement.rtf" />

For more context and details, please see this example of what minimal tweaks are needed to a fresh WiX project to get it to compile (see inline comments in WiX markup towards bottom).

2. Wix.dll: It might be that you have included a reference directly to Wix.dll in addition to WixUIExtension and WixNtFxExtension - both of which you need to keep included.

So in other words: remove the project reference to Wix.dll and try to recompile.

If that does not work, remove all references and re-add only WixUIExtension and WixNtFxExtension.

Some Suggestions & Links:

Similar issue: Where do I find Microsoft.Tools.WindowsInstallerXml.dll?
Maybe use a different source path than a temp directory.
Maybe exclude the *.pdb file from installation, unless you need it for debugging.
Use one file per component. IMHO this prevents all kinds of problems.
Strip your GUIDs from WiX sources before posting them - or they could be copy / pasted. Not good at all.