Use a calculation field (selected plans only) to trigger an error if the desired option is not chosen.
The calculation field has a function called ERROR("this is the provided error message").
When it is called, the form cannot be submitted, and the provided error message will become visible to the end user.
Calculation example:
Selecting the plus icon that appears on the right-hand side of the calculation input will allow you to select from your existing questions. This uses answer piping to display the submitter's response.
There are a few examples of preventing submissions in this template.
Gotchas
The calculation field must be visible to prevent the form from submitting.