Display warning based on cell value

Hello,

I have a drop down list that has values "yes"/"no" (cell F 20) and button in the next cell ( cell G 20).
If user, selects "yes", button will be visible which he could click to add "Sheets"
If he selects "no", button will be hidden.

My question : I have write a code that will throw a warning if user has selected "yes" and not added new sheet and it should revert the value from drop down to "no" in this case.

I am not sure, what to include in my code that will serve my purpose?

Code on worksheet

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Code on Module

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Thanks,
Sandu

Guys, I m still struggling with this. please suggest