Which sentence is correct?
The directory is not exist
The directory does not exist
This message informs a user about a folder he or she is trying to access.
The correct negation is:
Here, do functions as an auxiliary for emphasis/negation. Consider the positive form of the sentence where do is optional:
It should be clearer here that is is not appropriate:
Also, note that whenever be is used as an auxiliary, it precedes participles and infinitives. Hence, the following examples are grammatical:
Of your two examples, the correct sentence is
However, given your context, that is likely not very helpful to your user.
A more appropriate response would be to reference the folder directly: