Impeccable job or work

Let’s sat your boss gave you a task and she’s very demanding and before leaving she looks at you and says:

X: “I’m expecting you to do an impeccable ____”

Should it be job or work? What’s the difference?

As DW256 pointed out, strictly considering the example sentence, job is the only appropriate noun that would fit. However, if we simply remove the indefinite article an in the case where work is used, we have two possible sentences.

X: "I'm expecting you to do an impeccable job."

and

X: "I'm expecting you to do impeccable work."

As previously discussed here, a job and work are two distinct, but related, concepts. A job is a set task which is typically paid (as in the case of your example), whilst work is a broader term that can refer to any kind of labour or effort by a person, although it certainly does have a connotation of a job also ("What do you do for work?" as an example).

Applying this knowledge to your scenario, the correct phrasing depends on what the boss means. It is worth noting, however, that the two are likely broadly similar enough to be interpreted in a reasonable way by the recipient. If the boss states that she expects you to do an impeccable job, then it is implied that the overall product of your labour is what she is interested in, and whilst that likely means you will have to do impeccable work to achieve such a product, the key thing she is interested in is the end result.

By contrast, if she states that she wishes for impeccable work to be done, then both the process of creating the end product and the product itself are implied to be important, as work can imply both the labour done by an individual and the end result of that labour (i.e., the sum total of the work done).

As previously stated however, the difference in meanings between the two is fairly minor. Both can likely be used and have the same meaning to an individual, that what they do and what they produce as a result of their work must be of a particularly high standard.

Since you've used the indefinite article an, the only valid choice is job as work in this sense is a non-count noun.

If you were to take out an, then work would be the only valid choice as job is a count noun and requires a determiner in the singular case. Alternatively, you could use plural jobs, but that changes the interpretation of the sentence: she would have assigned multiple tasks. This also sounds awkward as we don't usually speak of doing good jobs, even if separate people do them. It'd be much more natural to say something like I expect you all to do a good job, even if the assignments are separate.

Other than that, there's not much difference between the two.