I'm pretty new to this, so please bear with me.

Here's the error message from AppCentre: :

Type 'Reading' is not known on line 2 in source list /etc/apt/sources.list.d/signal-xenial.list E: The list of sources could not be read

Also, when I click on the 'Installed' list, nothing comes up, except the icon for 'Operating System Updates'

Steam was the last app I installed, but that was a few days ago. Any help is much appreciated!