Uni student here, just about to finish their first year, so I can't be much help from a parenting standpoint but I do understand the fear about going to uni, especially as someone forced to distance learn because of Covid for their A-Levels so was lacking a bit in the socialising department.
First thing I'd say is to talk to them and try and gauge why they want to decline their offers -- are they not happy with the choices they have, do they want to change course, are they scared about getting into/starting uni or have they decided it just isn't for them? It might be they don't know themselves and are just a bit overwhelmed by it all, or there might be a specific reason behind it, and identifying that early is really important, especially since I know choice deadlines will be coming up. Maybe ask what changed since they seemed enthusiastic about it before.
In terms of addressing the potential fear of making friends, try to reassure them that it's not the end of the world if it takes a few weeks, and that you'll always be on the end of a phone call or a car/train/bus journey away if they get lonely. Maybe also let them know that there'll be lots of opportunities to meet people even before their course starts - I believe most universities have a freshers week, which is basically a full week of events both for students in general and course-specific ones. Even if they aren't the partying type - I have never been on a night out, not really my scene and certain circumstances prevent it - there are socials during the day, and fresher's fair can introduce them to societies they might be interested in joining, which are a good place to make friends with similar interests. For instance, half the people I know are from the rock and metal society I'm a part of. If they're in student accommodation, flatmates are also an excellent place to start talking to people - even just introductions as you're all moving in, setting up a flat group chat, maybe even organising a night to have a meal and get to know each other. I can't promise all flatmates are great, but you're bound to get at least someone you can chat to. There are so many opportunities, even before they have any lectures.
For me, I couldn't wait to move away from home, even temporarily, but that's mainly due to the environment I've grown up in. It's perfectly normal to be nervous about moving away, especially when there are so many responsibilities to take on. If they're worried about it, maybe sit down with them and make a plan of what they need to do before they leave, and what kind of things would need to be organised to help them settle in, e.g. work out where the supermarkets are, maybe teach them how to cook some basic meals and do meal prep and budgeting if that's something they need help with, just to try and make the process seem less stressful.
Whatever the case is, the best thing to do is to stay calm and listen to what they're saying. Make sure they've weighed up all their options before they make any decisions and know the pros and cons of each one, but don't force them into anything they're not comfortable with. That's not to say you can't offer advice - if anything, if they're feeling a bit lost at the minute, I'm sure they'll appreciate it.
This ended up being a bit longer than I intended, but I hope it helps and everything gets resolved