Step 1: Check for Another Server Instance
Make sure there is no other Minecraft server already running on the same machine.
Step 2: Find the Process Using the Port
Windows:
netstat -ano | findstr :25565
Mac/Linux:
lsof -iTCP:25565 -sTCP:LISTEN
Stop the process that is already using the port.
Step 3: Change the Port (Optional)
Edit server.properties:
server-port=25566
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Step 4: Restart the Server
After freeing the port or changing it, start the server again.