The phrase 'not at all possible' is correct and usable in written English.
You can use it to express that something is not possible or cannot be accomplished. For example, you could say: "I'm sorry, but completing the task in just a week is not at all possible."
It also makes ultraviolet observations not at all possible from Earth.
"No, it's not at all possible my stillbirth happened from the drugs," she said.
Is it very possible, somewhat possible, not very possible, or not at all possible?
Unfortunately it was not at all possible in the ensuing course of events to put into practice these good and reasonable ideas and statements made by Baldwin.
The doctors had stressed to him that "without an autopsy it is not at all possible to determine as to what had caused the injury," he wrote.
My nice kindly headmistress took my parents aside and said, 'I do hope you're not expecting university for Joanna and certainly not Oxford or Cambridge because it's really not at all possible'".
Try not to include more than 3-5 "must-haves" on your list, and no "not-at-all-possibles".
For non-Trumpians, taking pleasure in that spectacle was only possible if it was accompanied by a belief that a Trump Presidential victory wasn't at all possible.
This was particularly true since "under the relevant federal securities standards, it does not seem at all possible to have a compliant meeting on the time frame that the Icahn group suggests".
"The City Council would like to be clear that it considers it important that a city the size and status of Plymouth should not, if at all possible, be without a professional football club.
Much of the group's manpower remains, mostly because it made a point of fighting at a distance — or not at all — whenever possible despite the fury of the Israeli advance and bombardment.
