Translation:The box was not opening all the way.
"la scatola si apre" = the box opens. Box is the subject, "si apriva" is the action.(could not be opened further). Your phrase would have been: "la scatola non era aperta completamente.
It did not accept my translation "The box was not completely open." Does this Italian sentence give more the sense that the box could not be opened further, or only that it hadn't been opened further?
No, "apriva" is a verb, so the English translation must use open as a verb. Your sentence uses it as an adjective.
I tried to answer with "fully" as well. The box didn't fully open... it was rejected
I think it is a reflexive construction. There is no exact translation to English but it is something like:
La scatola, non, si . . , apriva , completamente. =
The box . , not, itself, opened, completely. ~
The box did not open itself completely. ~
The bod would not open completely. ~
The box was not opening completely.
The problem is the timeframe I think. This sounds like you're standing up next to a box and it didn't open whereas the past tense here (i think) means a box in the past would not open. Maybe they were able to open it after all.
Because in your translation you use "open" as an adverb not as a verb. (What was the box? The box was open)
This answer implies in English that the box was opening itself. A better translation would be, "The box could not be opened all the way."
Was not completely opened means the same thin and is more English than American!