How can I show an error message based on a condition?

have a date field and have the following critical

if "date of proof" <"today + 7"

       show new error message

       "scheduling available for a week"

endif

I'm not sure if I understand your question correctly.
But it seems that you want to show an error message based on a condition, is that correct?

Please check the following form: http://form.jotformpro.com/form/50674148216960 I created a condition in it to show an error message if the selected date is before "today+7"

Is this what you're trying to accomplish?

If so, feel free to clone the form to your account.

If not, please provide more details about what you're trying to do and we'll be happy to help.
Thank you.

I just want to add a new message and and join a field.

has the?

I apologize, but I don't understand your last post.

If what you want is to manually create a condition like the one you posted before, this is not possible, unfortunately. You need to use the conditional logic wizard to create conditions in your form.

Let us know if that's what you meant or not.
Thank you.

Follow the arrows in red in the attached image.

You have shown us Form Warnings settings, but what they can do is only to give you the option to change the warnings already set to be shown, not the ones that you would like to add.

To add a new warning you would need to create a field with the warning message, usually created with the Text element (under Form Tools):

Once you add it and style your error code, all that is left is to add couple of show / hide conditions to show the message and you can see how to set them up here: Smart Forms Using Conditional Logic

If you want to show or hide multiple fields at once you can also take a look at these guides:

Show or Hide multiple fields at once using form collapse tool

Hide or Show Multiple Fields at Once