The type or namespace name 'FlaxEngine' could not be found (are you missing a using directive or an assembly reference?)

Just in case someone don’t konw how to handle this.
To fix this,you need open the csproj file in your project,and navigate to

<ItemGroup>
    <Reference Include="FlaxEngine.CSharp">
      <HintPath>..\..\..\FlaxEngine\FlaxEditorLinux (1)\Binaries\Editor\Win64\Development\FlaxEngine.CSharp.dll</HintPath>
    </Reference>
  </ItemGroup>

And replace “Win64” to “Linux”, looks like below

<ItemGroup>
    <Reference Include="FlaxEngine.CSharp">
      <HintPath>..\..\..\FlaxEngine\FlaxEditorLinux (1)\Binaries\Editor\Linux\Development\FlaxEngine.CSharp.dll</HintPath>
    </Reference>
  </ItemGroup>