I am wondering if there is any difference between the following sentences with different-placed not.
- I should not have been here today.
- I should have not been here today
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There's no difference between the meanings, but your second example would likely never be used by an educated native speaker.
The problem is that "have been" is the present perfect continuous form of the verb "to be." In other words, the two words belong together to create one compound verb. They should never be split apart.
Therefore, while the meanings of the two examples are the same. #1 is grammatically correct and #2 is not.