Maybe Works

module.exports.run = async (bot,message,args,db) => {
    if(message.deletable) await message.delete();
    let prefix;
    db.collection('guilds').doc(message.guild.id).get().then((d) => {
        prefix = d.data().prefix;
    }).then(() => {

        let xEmbed = new Discord.RichEmbed()
        .setTitle("Wrong usage :x:")
        .addField("Correct usage:", `${prefix}setprefix <new prefix>`)
        .setColor("d4ff00")
        .setTimestamp(new Date());

        if (args.length === 0){
            message.channel.send(xEmbed);
        } else if (args.length === 1){
            let nPrefix = args[0];


        let yEmbed = new Discord.RichEmbed()
        .setTitle("Successfull :white_check_mark:")
        .addField("New prefix set to:", nPrefix, true)
        .addField("New prefix set by:", message.author, true)
        .setColor("d4ff00")
        .setTimestamp(new Date());

        db.collection('guilds').doc(message.guild.id).update({
            'prefix' : nPrefix
        }).then(() => {
            message.channel.send(yEmbed);
        });
    }

}



}

module.exports.help = {
    name: "setprefix"
}