My server doesn't load when I try to run a custom map?

I am trying to run a custom map on my server. I've watched many video tutorials and it seems that I am doing nothing wrong. The server only loads with the default world but as soon as I put my map into it and change the 'level-name=' it says: "Failed to check session lock, aborting". I've been trying to look online for the solution for the past couple hours but found nothing. What can I do to fix this?