|There is no formula that will turn a Drop Down list on and off.|
It could done with VBA but that comes with it's own issues.
One possible solution is to use Named Ranges to populate the Drop Down from two different sources, one that has your choices and one that only says Not Applicable.
Place the list you want to use for Yes in a range. Name the Range Yes. Put the words Not Applicable in another cell and name it No.
The source for your Drop Down should read =INDIRECT(A1)
When A1 contains Yes the Drop Down will have your choices.
When A1 contains No Not Applicable will be the only choice.
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