The expression 'just now' has two different meanings. When it is used to mean "at this particular moment", it is usually a reference to something that is only temporarily true /something that is currently true, but only for a very limited time.
For example, if you call me and ask me to go shopping with you now, I might tell you that I am in the middle of cooking dinner, so I can't go anywhere "just now" (but probably I will be able to go in with you an hour or two from now).
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