rsvp submission without picking a meal

This will happen regardless if it’s a physical card or online. It’s a faux pas on your part as a bad host to prevent them from attending if they don’t pick their meal. You need to be gracious and reach out to them kindly to get their response. The same will apply for those who didn’t write their name or they didn’t respond at all for whatever reason. You reach out and be kind to them when you talk to them.

View Quoted Comment I appreciate your response, but I’m talking about the website functions. They should put on some way for people to not be able to submit the online RSVP without completing all necessary info.

While frustrating that they could follow simple instructions, you'll need to just call them and kindly say that you saw they forgot to select their meal and you'd like to know what they would like. Don't make it more complicated than it needs to be.