Why it keep saying *Rookit*(virus) from AVG SANDBOX?

Hello Mat, 

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Everytime i do scanning in AVG, it keeps popping up rootkit in AVG sandbox, what does it mean and how can i remove it?

Version of my AVG Antivirus (21.7.3196 (build 21.7.6523)) It is a free version of AVG Antivirus. thanks for helping!

Version of my AntiVirus 21.7.3196 (build 21.7.6523.689)

It is a free AVG Antivirus. 

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