The 500 error page is being generated by your origin so check the logs there to see which requests are causing that to be returned and debug from there.
You will only debug what is happening by checking your origin to see why it is returning 500 for requests going via Cloudflare and what it doesn’t like about them.
Do you know how I can do that ? I would be grateful for a little help ^^ Apart from running it on my computer on don’t know how to do anything apart from that
Are you using your own server or a host? If a host, which one? It may be they don’t like the Cloudflare proxy (although 500 is not the usual error in that case).
You can try setting the DNS record to “DNS only” which will tell you if the server is working ok on the application when deployed but with requests not passing through Cloudflare.