The path "C:/Users/Abigail.Desktop" on a Windows computer is typically the root path to your user files, assuming your username for the machine was "Abigail.Desktop". The error message implies that it's looking for the add-on files in that folder, which would be extremely unusual for them to be there. On Steam, the default install path would be "C:/Program Files (x86)/Steam/steamapps/common/SOMA", and any add-on you download from the Workshop would be somewhere like "C:/Program Files (x86)/Steam/steamapps/workshop/282140/<add_on_id_#>".
Could you try to load the add-on again? If it fails again, reply here with your "hpl.log" file attached. (Instructions on where to find it are at the bottom of
this troubleshooting guide.)