Dealing with bugs I can’t reproduce

I've been assigned a bug at work that I'm having difficulty reproducing. I've tried all possible flows but I can't make the bug reappear again. I'm not using any testing library ( when I say \"I\", I mean that the team itself hasn't written any tests ) and everything is done manually though there is a cypress configuration in place, lying dormant. How do I deal with such a bug? Also, what is your flow like when dealing with bugs you've been assigned: Do you write a test first, or run the application and test it manually…?