Can't submit the form?
Do you have an Opt-in or Terms and Conditions field that is mandatory? Check to make sure the colours of the form aren't obscuring when the checkbox is ticked / unticked, and that you have set some appropriate text copy for the "What message should we show if someone doesn't tick to opt in?" field. This can make it appear that a form can't be submitted, but in reality the mandatory checkbox just hasn't been selected.
If someone has already submitted the form and tries to do it again, there is no error message appearing, just the red box like below:
It is probably because you have not updated the placeholder message in the form settings. Go to the Form > Advanced Settings > Check 'If contact already exists, show this message:' field. If the character count shows 0/100, it means that this is still the placeholder message. Enter your own message, or even copy and paste the exact same one that is already there and it will fix it.