Hello @assia855, welcome to the Julia community!
Regarding your question, it would be possible to help you if you provide more information. What exactly you did in your program before you get this result? What packages you were using? The best way to help us help you is to provide MWE, i.e. Minimal Working Example. Just a few lines, that someone can also run in their REPL and get the same error message. Also, please remember that MWE should not include data that is only in your possession.
You can read more about MWE and some other simple things that can ease your life and get answer quickly here: Please read: make it easier to help you