Omnisharp-vscode: C# IntelliSense failing to load

[hamhub7]

Issue Description

When I launch a C# script, there is a pop-up in the bottom right that says "Some projects have trouble loading. Please review the output for more details." I'm using this with Unity and I'm unable to see any autocomplete things besides words that I have already typed in the document.

Steps to Reproduce

This happens all the time, whenever I boot up VS Code.

Expected Behavior

Autocompletion of functions and variables from Unity.

Actual Behavior

No autocompletion or suggestions at all besides built in functionality.

Logs

OmniSharp log

Starting OmniSharp server at 8/11/2019, 12:12:46 PM
Target: d:Udemy RPGUdemy RPG.sln

OmniSharp server started.
Path: C:UsersAaron.vscodeextensionsms-vscode.csharp-1.21.0.omnisharp1.34.0OmniSharp.exe
PID: 9380

    Starting OmniSharp on Windows 6.2.9200.0 (x64)
    DotNetPath set to dotnet
    Located 4 MSBuild instance(s)
        1: Visual Studio Build Tools 2017 15.9.28307.770 - "C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\MSBuild\15.0\Bin"
        2: Visual Studio Community 2019 16.2.29123.88 - "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin"
        3: Visual Studio Community 2017 15.9.28307.770 - "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\MSBuild\15.0\Bin"
        4: StandAlone 16.0 - "C:\Users\Aaron\.vscode\extensions\ms-vscode.csharp-1.21.0\.omnisharp\1.34.0\.msbuild\Current\Bin"
    Registered MSBuild instance: Visual Studio Community 2019 16.2.29123.88 - "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin"
    Detecting Cake files in 'd:\Udemy RPG'.
    Could not find any Cake files
    Detecting projects in 'd:\Udemy RPG\Udemy RPG.sln'.
    Queue project update for 'd:\Udemy RPG\Assembly-CSharp.csproj'
    Detecting CSX files in 'd:\Udemy RPG'.
    Could not find any CSX files
    Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.CSharpFormattingWorkspaceOptionsProvider, Order: 0
    Loading project: d:\Udemy RPG\Assembly-CSharp.csproj
    Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.RenameWorkspaceOptionsProvider, Order: 100
    Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.EditorConfigWorkspaceOptionsProvider, Order: 200
    Configuration finished.
    Omnisharp server running using Stdio at location 'd:\Udemy RPG' on host 19688.
    The reference assemblies for .NETFramework,Version=v4.7.1 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks
    Failed to load project file 'd:\Udemy RPG\Assembly-CSharp.csproj'.

d:Udemy RPGAssembly-CSharp.csproj
C:Program Files (x86)Microsoft Visual Studio2019CommunityMSBuildCurrentBinMicrosoft.Common.CurrentVersion.targets(1175,5): Error: The reference assemblies for .NETFramework,Version=v4.7.1 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks

    Attempted to update project that is not loaded: d:\Udemy RPG\Assembly-CSharp.csproj

C# log

Nothing shows in this tab

Environment information

VSCode version: 1.37.0
C# Extension: 1.21.0

Dotnet Information

.NET Core SDK (reflecting any global.json):
Version: 2.2.401
Commit: 729b316c13

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18362
OS Platform: Windows
RID: win10-x64
Base Path: C:Program Filesdotnetsdk2.2.401

Host (useful for support):
Version: 2.2.6
Commit: 7dac9b1b51

.NET Core SDKs installed:
2.2.401 [C:Program Filesdotnetsdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.2.6 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.2.6 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.App]
Microsoft.NETCore.App 2.2.6 [C:Program FilesdotnetsharedMicrosoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

|Extension|Author|Version|
|---|---|---|
|code-gnu-global|austin|0.2.2|
|cpptools|ms-vscode|0.25.0-insiders|
|csharp|ms-vscode|1.21.0|
|discord-vscode|icrawl|3.8.1|
|python|ms-python|2019.8.29288|
|remote-wsl|ms-vscode-remote|0.39.2|
|team|ms-vsts|1.149.2|
|vscode-markdownlint|DavidAnson|0.30.1|;

[rchande]

@hamhub7 Your log includes the error The reference assemblies for .NETFramework,Version=v4.7.1 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks. Can you try installing the 4.7.1 developer pack from the link?
https://aka.ms/msbuild/developerpacks

[andsynchrony]

Installing Mono helped me. Seen here

[cartermp]

@rchande is correct here. Mono can also help if it's the right version and brings along the appropriate reference assemblies. Once this is installed, there shouldn't be any issues.