"Predefined System.Object is not defined or imported" every time I upgrade Unity

Today I upgraded Unity to 2019.1.a, and as expected, when I open Visual Studio nearly every line of code is underlined. In addition, the project compiles fine and I can run the game in the Editor. I went through the motion of completely removing and installing Visual Studio, but the errors persisted. What should I do to make the useless errors disappear? IMGUR link :
https://imgur.com/a/7N0dh1u

 

I tried building the project and found this error:

/Library/Frameworks/Mono.framework/Versions/5.16.0/lib/mono/msbuild/15.0/bin/Microsoft.Common.CurrentVersion.targets(5,5): Error MSB3644: The reference assemblies for framework ".NETFramework,Version=v4.7.2" were not found. To resolve this, install the SDK or Targeting Pack for this framework version or retarget your application to a version of the framework for which you have the SDK or Targeting Pack installed. Note that assemblies will be resolved from the Global Assembly Cache (GAC) and will be used in place of reference assemblies. Therefore your assembly may not be correctly targeted for the framework you intend. (MSB3644) (Assembly-CSharp)

 

Do you know how to fix this, im running into this after upgrading to 2019.1