I feel for her, and you. It is a horrible situation to be in.
When DS was is a serious rut that was dipping into severe depression, I helped him out of it by gently coaxing him to build a fuller life, so that the lack of the one thing that mattered to him (in your daughter's case, finding a job) left only a small gap, not a gaping hole.
Make a list of things she can control. She can control her own health and wellbeing. Suggest she steps back from job apps for now and spends at least two months focusing on her own wellbeing - a really nutritious diet, gentle exercise, time outdoors in nature, some therapy or self help, and once she feels up to it, a bit of life-development. I got DS to make a list of 9 areas of life that matter and to do something to improve at least one of them - however small - every day. And keep a record of progress. Once she is taking good care of herself physically, she can branch out and start redoing something she used to love - playing an instrument or singing - joining an art class or sports club or choir. Once that is part of her life she could look into either volunteering - it doesn't have to be a charity - could be reading with children at a local school or brownie group, helping at church if you are religious, joining a community gardening scheme - anything that makes her feel like her contribution matters.
Then I strongly suggest, as soon as she is feeling a tiny bit brighter, she does something - anything – to create her own work. That could be offering Spanish tutoring and conversation classes to people wanting to go to Spain on holiday. Or she could read a load of marketing books to make sure her degree info is up to date, and offer small local businesses some marketing strategies on how to grow their profile. Or even set up as a dog walker, if she likes animals. In this way, she has something - or a few things - on her CV. She could even start a blog or substack about things that interest her, that are related to the profession she wants to go into - just commenting on marketing campaigns and strategies - giving her considered opinion. That could be a good thing to have on her CV.
Get her to go back to dreaming big not small about what she wants to do in life. When she has a clear picture of a career she might enjoy - think of every single person you know to see if any of them have a connection with that world, and then approach them directly and ask to shadow them. Or offer her services free for a couple of weeks to a local small business or start up.
This is what DS did. He did one day a week, unpaid, as an intern in a small business that really needed him. He didn't feel exploited as he was gaining genuine experience, not just shadowing people and doing coffee runs. That led to pay internship and then his first job.
It is hard, but the people I know who thrive just created work for themselves. DS did this too. He set up his own small business in the profession that he wanted to enter, created a blog, so that along with the free and paid internships, he had a visible track record in his chosen field. And if she does stuff for herself, that covers the gap on her CV, so it shows continuity.
Good luck to her.