I love using repl.it for creating node express server but I'm having the following problems:
At one point it cannot start, saying port 3000 is already in use and there is no way to kill it. What I do is duplicate the repl and go from there :/
My server goes down after some time so I want to ask if there's a good reliable host for my node server
As for issue 1, try not passing any port number to allow it to use any available ports. You can also fork your repl and run the new one after deleting the old one to kill the process.
As for the second issue, you would need to set up a pinging service such as uptimerobot or https://portal.kognise.dev/ping which send an http request to your server at fixed intervals, be sure to keep the interval below 30 minutes. I also showed how to setup uptimerobot in this tutorial.