Cannot install updates Ubuntu 14.04

I get a message which says "Package Managers cannot be resolved" when I attempt to install regular updates.

When I try to install from the Terminal with sudo apt-get update I get the following:

Reading package lists... Done
W: There is no public key available for the following key IDs:
1397BC53640DB551
W: Ignoring Provides line with DepCompareOp for package libreoffice-l10n
W: Ignoring Provides line with DepCompareOp for package libreoffice-l10n
W: Ignoring Provides line with DepCompareOp for package libreoffice-l10n
W: Ignoring Provides line with DepCompareOp for package libreoffice-l10n
W: Ignoring Provides line with DepCompareOp for package libreoffice-l10n
W: Ignoring Provides line with DepCompareOp for package libreoffice-l1

I can't seem to install updates anymore. What is the issue? I am using Ubuntu 14.04.5 LTS.