Package manager error (access denied) on Mac

Each new project on a MacBook Pro that is created comes with the following error:

If I continue, I can get into the project, however there are no base packages imported (such as the physics 2D one), which is making progress in development impossible! I am working with version Unity 2018.3.0f2 of the unity app.

I am very new to unity and have searched everywhere for a solution to this issue, so I apologise if I have missed a thread which answers this. Any help on rectifying this would be fantastic and a real stress reliever after many hours of frustration.