I have High Sierra. When ever I click on iTunes I get a message that iTunes can't verify the server "s.mzstatic.com", eventually I do get into my iTunes library, my WEBROOTSecurity list this "s.mzstatic" as a Trojanthreat,

The free Malwarebytes may take care of it...

https://www.malwarebytes.com/mac/

See if you have unknown Profiles.

To remove a configuration profile in macOS:

1. From the Apple menu, select System Preferences....
2. From the View menu in System Preferences, select Profiles.Note:
3. Profiles won't be visible until you have at least one profile installed.
4. Select the profile you want to remove, and then press the - (minus) button. Click Remove to remove the profile.