The server keeps restarting
What it looks like in your log
Starting minecraft server version 1.21.1 (repeated every few minutes)
What causes it
This log covers several separate start-ups. Something is ending the server shortly after each boot and something else is starting it straight back up.
How to fix it
Read the end of a cycle rather than the start of the next one: find a start-up line and read upwards from it. If there is no error there at all, the server is being killed rather than crashing, and memory is the first thing to check; a restart loop with nothing in between is what an out-of-memory kill looks like from inside the log.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log