I see there's no reply so i'll try to explain even tho im not a native speaker. This tense is used to describe states in the past. Being somewhere isn't an action you just do, it's more of a phase. It describes the past, or in a more figurative way, what the past looked like when it was still the present - not just what action was being held. Therefore "ero" is the right form.
The first time I translated it, I wrote, "I was here last evening" It was deemed incorrect, giving the correct translation as, "I was here last night." The second time I translated as, "I was here last night" and it was correct but with the added info that it was also correct to say "I was here yesterday evening"!?! WTH??!!
In English "last evening" is odd. It would be "yesterday evening" or "last night". Please refer to: https://englishtips.quora.com/Last-night-yesterday-evening-and-more
DL is using this and "sono stato qui" for the same english translation of being physicall present somewhere. Which is correct? I know the difference between "stare" + adj and "essere"+ adj, the latter conveying a sense of that person's character, but i can't wrap my head around the difference between "stare" + location and "essere" + location.