"MonoBehavior Could Not Be Found" Appears When Attempting to Write a Script in Unity

When I started a new project with Unity and tried to create a script,
Monobehaviour not found, it will be .

This has never happened before, but after changing the computer and changing the version,
Once you uninstall and re-download Unity, it will remain in this state.
...I'm an amateur and I don't know why and what to fix

Run Environment
Mac OS X (10.14.4)
Unity for mac ver 2019.1.3 f1 personal
visual studio ver8.0.8(build2)

error message

Error: Could not find type or namespace name "System".
  (Verify that the using directive or assembly reference is specified.)
   The using directory is not required.

All of these engine related sections are in error.
I came here hoping you'd give me some advice.
Thank you for your cooperation.

Notes
skates the unity version.