Online Mode
If at anytime you need to disable the server's online-mode setting to authenticate with the Mojang servers, you can do so by navigating to Files > Config Files > Server Settings in the Multicraft control panel. Then set the "Online Mode" setting to Disabled. Hit save at the bottom and restart your server for the changes to take place. Note that by disabling online-mode, you are essentially turning your server into a "cracked server" where users without a legitimate copy of Minecraft can join your server, which does increase the odds that they are using cheats/hacks
