Hi Emilia,
It happens. People get cold feet all the time when it comes to first dates, particularly with someone they initially met on a dating site. Don’t beat yourself up so much.
Just let him know that you are grateful that he asked you out but you don’t believe that there is a romantic connection, and that you wish him the best, but you won’t be able to come to the date.
Being polite, but also honest with a guy, is good for the both of you. It is kind and reasonable because it doesn’t lead him on. It allows him to accept your disinterest with dignity and move on. Leading him on, making him believe that there is romantic potential, is way worse.