Open the resource configuration.
Add a Reaction.
Enter a name.
Select the reaction type — Show warning.
Enter the text of the message and click Add condition.
In the list, pick the field By car with the Checkbox is checked condition.
Then the warning will be displayed when the By car checkbox is set by the customer.
The error text is displayed with an orange line.
It is shown at the screen when you select options and on the cart screen.
See the result: