I cannot enable the USB debugging icon

Hi, I am a newcomer and hope to find my answer here. I have a LG-E405 and I know that it is embracing
I want to enable it into USB debugging, from this direction: Setting>Application Setting>Development, But the problem is that I cannot to enable the USB debugging icon. Because it is dark. I donot know what do you say to this situation, but I see that this icon is not light, like other, and so I cannot enable it. What do you think? Please help. Thank you so much.

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