There is hope yet !
You are advised of an action to take:
Take a look and correct your "/etc/apt/sources.list" file and/OR the files in the 3rd party directory "/etc/apt/sources.list.d".
Release file (Wrong sources.list entry or malformed file)
If you want confirmation, post the outputs - between code tags, please - of terminal commands:
code tag tutorial:
Once you are sure the source index files are good, test:
Depending on those outputs, to insure the upgrade process completes; will require a few other terminal commands.
sudo apt-get update
sudo apt-get upgrade
I am hesitate to make the further advisement until the state of the package manager and installed software is known - the update/upgrade process outputs.
ain't nothing but a thing