It seems there is a problem with asmdef files in Unity 2018 with Visual Studio 2017, you'll need an update.

https://developercommunity.visualstudio.com/content/problem/88183/error-when-unity-launches-visual-studio-2017-to-ed.html

https://developercommunity.visualstudio.com/content/problem/130597/unity-intellisense-not-working-after-creating-new-1.html

https://forum.unity.com/threads/issues-with-visual-studio-unity.514304/

You can try the following:

• Update your Visual Studio 2017 to the latest version. (v15.7.1)

This will most likely solve your problem. If not:

• Delete asmdef files from packages folder of Unity. But this may cause your project to not compile if you have a dependency on those packages. The folder should be in C:\Users{UserName}\AppData\Local\Unity\cache\packages\packages.unity.com

It seems there is a problem with asmdef files in Unity 2018 with Visual Studio 2017, you'll need an update.

https://developercommunity.visualstudio.com/content/problem/88183/error-when-unity-launches-visual-studio-2017-to-ed.html

https://developercommunity.visualstudio.com/content/problem/130597/unity-intellisense-not-working-after-creating-new-1.html

https://forum.unity.com/threads/issues-with-visual-studio-unity.514304/

You can try the following:

• Update your Visual Studio 2017 to the latest version. (v15.7.1)

This will most likely solve your problem. If not:

• Delete asmdef files from packages folder of Unity. But this may cause your project to not compile if you have a dependency on those packages. The folder should be in C:\Users\ {UserName}\AppData\Local\Unity\cache\packages\packages.unity.com