It doesn't exist - correct It is not existing - incorrect

So existing never can be used?

So existing is not correct at all?

“Isn’t existing” doesn’t exist😅😅

It doesn't exist = whatever x is does not exist. When it is not existing is an unusual construction. It sound like something that might only be used in science or physics or a logic puzzle, more context is needed. Not there is not the same as not existing. https://dictionary.cambridge.org/dictionary/english/exist https://dictionary.cambridge.org/dictionary/english/there exist to be real or alive there to be at in a place