If you are talking about work in general, then "at work". However, working on a particular task "on the job".

You can’t be ‘in the job’ as in means to be inside something, like a building, or a car, or whatever. I’m assuming you are looking for a phrase that means ‘at work’, if so ‘at the job’ doesn't work either. It could work if you use ‘job’ to mean something like a job to be viewed, I can give you an example but I don’t think that is what you are asking. The simplest way to put what you want to say is ‘at work’.

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