The usual word for this is "ошибка" which is hard to spell on 7seg, but the main reason was that designs were copied layer by layer from chips and not reverse engineered (with few rare exceptions) to be able to change something like that.
What’s lost to history is the lore of Soviet calculator design. These were made by real people who made design decisions based on the constraints they had, even when it came to cloning Western tech.
Just like Latin, some Cyrillic characters can be displayed, but the words that mean an error, unambiguously, have letters that don't fit, even if you shorten the word. Ошибка (ш & к are problematic), сбой (й won't fit).
Best I can come up with is OLL|6, a crude rendering of ОШБ, abbreviation of “ошибка”, error. Or maybe ПРOБ, as “prob” in “problem”
I see several words which could fit but all of them are obscene.