It's forbidden to proceed, until the task hasn't been completed. Task is a singular noun and for that we need to use 'hasn't. The sentence is a little bit out of context, maybe try to change the tense, maybe a future form, like: It's forbidden to proceed, until the task won't be completed/will have been completed.
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AnonymousIt's forbidden to proceed, until the task hasn't been completed.Wrong, - until the task has been completed.