Well, I suppose I expected that you had noticed the other conversations in the forum where byzantium (byz) backports (in this case shorthand for “additional repository of more updated software that’s not the official PureOS repository where everything else comes from”) were mentioned. It’s a way to keep some stuff current and working since software updates, due to tangled inter-dependencies, get borked every now and then, or just not offering the latest, when time passes, as PureOS version named Byzantium is getting a bit old (as it’s based on Debian 10 - and Debian already having more current versions [hence the other threads for Crimson]). More official explanation here and more here.
Don’t worry, you can always do a web search if a term is unfamiliar - I’ve looked up stuff at least thrise just today that I wasn’t sure of. L5 is… “an adventure” for sure 
Anyways, if you want or dare to add the backports repo as instructed as a source you want/trust/need enough, have a look at the thread from around January. As you can see, things can go wrong too. If you’ve done jumpdrive and/or flashing, I’d say it’s a bit easier, but I’d do first a jumpdrive connection and get a disc partition image (using external linux computer’s Discs app) as a backup, so you’ll always be able to return to exactly the same state you were. Just to be on the safe side (and it’s good practice even if something else went wrong with the phone). If this sounds too daunting, don’t do it, just wait for the official repo (or Crimson) to get updated. Know that there really is an app that you were missing (instead of it remaining just a wish - if that’s any consolation).
I have no idea what “E” is, so we have that in common. Maybe “error”. But that’s not the relevant part of that message.