Sudden message appearing on the screen over and over

What kind of message? Adware? Pop-ups or something else?

1. Click  menu in the menu bar, hold the  shift key on the keyboard down and choose “Force Quit Safari” from the dropdown.

    Safari will quit.

    Launch Safari holding the shift key down.

         https://support.apple.com/en-us/HT201276

 2. For more info: Pop-ups and ads in Safari: https://support.apple.com/en-us/HT203987

3. Check for adware.

     Run the latest release of Malwarebytes for Mac to remove malware/adware, if installed on your Mac.

     For instructions: Install Malwarebytes for Mac v4      Uninstall Malwarebytes for Mac

     Click the “FREE DOWNLOAD” button.

     Click the “Scan ” button. Once done, quit Malwarebytes for Mac.

     Restart the computer and relaunch Safari holding the shift key down.

     Scan for Malware again.

4.  If this doesn’t help:

     Download EtreCheck: https://etrecheck.com/,run it and post the report here.

     Click  “Free Download” button,    

     Open Downloads folder, click on it to open, and then select ”Open”.

     “Choose a problem” from the popup menu box, and then “Start EtreCheck” in the dialog.

     Click “Share Report” button in the toolbar, select “Copy report” .

     Paste the report when you reply. This is a diagnostic test.