As of Minecraft 1.13, Mojang have changed the location where the Minecraft server is to be downloaded from. This means that both McMyAdmin 2 and AMP need updating to accommodate this.
AMP users can either upgrade to the latest Nightly build (as of July 21st 2018), or download the server manually from Mojangs Server Download page, upload it by drag-dropping into AMPs file manager and telling AMP to use the new jar file.
McMyAdmin 2 users with a Pro licence should consider upgrading to AMP, but if that's not possible you will need to rename the server to
minecraft_server.jar and upload it.
In McMyAdmin 2 sleep mode is also not available at this time for 1.13, but it does function correctly in AMP.