“Failed to unmount ‘/data’ (Device or resource busy)” What does that mean?

Hello everybody, thank you to have clicked on my topic :D

I explain my problem:
I wanted to do a nandroid backup, before installing a rom on my P9 with TWRP(version 3.1.1-0), I have selected “Boot,” “System,” and “Data” (as usual)
But a message appeared: “Failed to unmount ‘/data’ (Device or resource busy)”
What does that mean?
How do you solve that? Is it a problem?
I can restore without “Data”?

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