What part of “office building, at night” did you not understand?
And, no, it is not a requirement under international law to make sure that no civilians get hit. If grandma brings grandson a cake to base and the base gets blown up she’s collateral damage, what matters here is that the target was military and that reasonable precautions have been taken to spare civilians – in this case, to do it at night.
Otherwise, if you’d say “you can’t ever attack within a city” everyone would have their military HQs etc. dispersed throughout residential areas and, consequently, nobody would be willing to spare civilians. Is that what you want?
If anything you should be complaining about Russia (attempting to) use civilians as shields by dispersing their operations exactly like that.