I need help in HTML and CSS

Hello, I have a question: If I want to arrange the cards side by side with "display: flex;" the cards disappear. If you remove the command, the cards are back. I once hosted a website for this because the code is a bit longer: https://64875e983404d208a5532d37--silver-kulfi-ebccf7.netlify.app/ I would be very happy about your help, thank you.