I see you’re using the syntax for Meta Box Conditional Logic instead of Meta Box Show Hide. This code is created with Meta Box Builder, which only supports Conditional Logic. So to use Show Hide, please modify the syntax to match the rules here.
If you want to use Conditional Logic, please note that the taxonomy value is an array, so you should use
contains, like this:
array( 'tax_input[product_cat]', 'contains', 18 )