You had something wrong but i found it and i fixed it

// Requires: PushAPI

using Oxide.Core.Libraries.Covalence;
using Oxide.Core.Plugins;

namespace Oxide.Plugins
{
    [Info("Call Push", "Wulf/lukespragg", 0.1)]
    [Description("Send a push notification when players call for admin")]
    class CallPush: CovalencePlugin
    {
        [PluginReference]
        private Plugin PushAPI;

        [Command("calladmin")]
        private void CallAdmin(IPlayer player, string command, string[] args)
        {
            if (args.Length <= 0)
            {
                player.Reply("Please enter a message when calling for admin");
                return;
            }

            player.Reply("Calling for admin!");
            PushAPI?.Call("PushMessage", $"{player.Name} is calling for admin", string.Join(" ", args));
        }
    }
}​
why you dont no public this like extension for PushAPI?