Piotr, I am an IT professional so I understand the subject you are talking about. However your sentence is not clear. I think you are missing a verb. Do not ______ another ... One possible sentence is ... Do not correct another bug until you get the first trouble ticket closed. See the verb "correct". This makes the sentence more clear. If "bug" was already understood, you could say... Do not fix another until you get the first one fixed. Another possibility is... OLD: Do not another that what you'll be maintaining. NEW: Do not fix another bug in the code you'll be maintaining.

Sorry, but it's not very clear what you're trying to say with this sentence. Can you explain what you mean, please?

no, it's not

Maintaing code means: "Basically fixing bugs that are reported, keeping up to date with any external dependencies (libraries), etc." So finally which version is correct?

I still don't fully understand it, but yes, I guess you could say that, if it makes sense in a programming context.