In the Quizk FBT offer menu, you will find this option under Variant Selector and by default the selected option is Pre-select first variant.

This means the first variant in the offer if by default selected in the bundle but if you untick this option you have the option to customize the message your customers see on the variant drop-down menu and the error message that would appear if they don't select anything and just claim the bundle offer.

In the store, if they were to claim a bundle offer for which this option is selected, a popup message will show at the top of the tab. The text on it will be represented by the "Error Text" that you input.