How can i add a rank check to this script?

I'm working on a script for dj's to play music but i have no idea how to add a custom check to it and if there is anything wrong with the script, please let me know.

UnrefinedRank = player:GetRankInGroup(GroupId)
set['Prefix'] = "-"
if a=='Prefix' then
    local orig=set.Prefix
            set[a]=b
            for i,v in pairs(set.Commands) do
                if v.Prefix==orig then
                    v.Prefix=set.Prefix
end
set.ScriptMusicList={
{n='heybrother',id=143786134};
{n='loseyourself',id=153480949};
{n='diamonds',id=142533681};
{n='happy',id=146952916};
{n='clinteastwood',id=148649589};
{n='freedom',id=130760592};
{n='seatbelt',id=135625718};
{n='tempest',id=135554032};
{n="focus",id=136786547};
{n="azylio",id=137603138};
{n="caramell",id=2303479};
{n="epic",id=27697743};
{n="rick",id=2027611};
{n="crystallize",id=143929751};
{n="halo",id=1034065};
{n="pokemon",id=1372261};
{n="cursed",id=1372257};
{n="extreme",id=11420933};
{n="harlemshake",id=142468820};
{n="tacos",id=142295308};
{n="wakemeup",id=147632133};
{n="awaken",id=27697277};
{n="alone",id=27697392};
{n="mario",id=1280470};
{n="choir",id=1372258};
{n="chrono",id=1280463};
{n="dotr",id=11420922};
{n="entertain",id=27697267};
{n="fantasy",id=1280473};
{n="final",id=1280414};
{n="emblem",id=1372259};
{n="flight",id=27697719};
{n="banjo",id=27697298};
{n="gothic",id=27697743};
{n="hiphop",id=27697735};
{n="intro",id=27697707};
{n="mule",id=1077604};
{n="film",id=27697713};
{n="nezz",id=8610025};
{n="angel",id=1372260};
{n="resist",id=27697234};
{n="schala",id=5985787};
{n="organ",id=11231513};
{n="tunnel",id=9650822};
{n="spanish",id=5982975};
{n="venom",id=1372262};
{n="wind",id=1015394};
{n="guitar",id=5986151};
{n="selfie1",id=148321914};
{n="selfie2",id=151029303};
{n="fareast",id=148999977};
{n="ontopoftheworld",id=142838705};
{n="mashup",id=143994035};
{n="getlucky",id=142677206};
{n="dragonborn",id=150015506};
{n="craveyou",id=142397454};
{n="weapon",id=142400410};
{n="derezzed",id=142402620};
{n="burn",id=142594142};
{n="workhardplayhard",id=144721295};
{n="royals",id=144662895};
{n="pompeii",id=144635805};
{n="powerglove",id=152324067};
{n="pompeiiremix",id=153519026};
{n="sceptics",id=153251489};
{n="pianoremix",id=142407859};
{n="antidote",id=145579822};
{n="takeawalk",id=142473248};
{n="countingstars",id=142282722};
{n="turndownforwhat",id=143959455};
{n="overtime",id=145111795};
{n="fluffyunicorns",id=141444871};
{n="gaspedal",id=142489916};
{n="bangarang",id=142291921};
{n="talkdirty",id=148952593};
{n="bad",id=155444244};
{n="demons",id=142282614};
{n="roar",id=148728760};
{n="letitgo",id=142343490};
{n="finalcountdown",id=142859512};
{n="tsunami",id=152775066};
{n="animals",id=142370129};
{n="partysignals",id=155779549};
{n="finalcountdownremix",id=145162750};
{n="mambo",id=144018440};
{n="stereolove",id=142318819};
{n='minecraftorchestral',id=148900687}}
coroutine.wrap(function() for i,v in pairs(set.ScriptMusicList) do table.insert(set['MusicList'],{Name=v.n,Id=v.id}) end end)()
set.MakeCommand('Start Music',2,set.Prefix,{'music','song','playsong'},{'id'},1,function(plr,args)
coroutine.wrap(function()
for i, v in pairs(game.Workspace:children()) do if v:IsA("Sound") then v:Destroy() end end
local id = args[1]:lower()
local pitch = 1
local mp = game:service('MarketplaceService')
local volume = 1
for i,v in pairs(set['MusicList']) do
    if id==v.Name:lower() then id=v.Id end
    if v.Pitch then pitch=v.Pitch end
    if v.Volume then volume=v.Volume end
end
local name = 'Invalid ID '
pcall(function()
    if mp:GetProductInfo(id).AssetTypeId==3 then
        name = 'Now playing '..mp:GetProductInfo(id).Name
    end
end)
local s = Instance.new("Sound", game.Workspace) 
s.SoundId = "http://www.roblox.com/asset/?id=" .. id 
s.Volume = volume 
s.Pitch = pitch 
s.Looped = true 
s.archivable = false
pcall(function() s:Play()end)
set.Hint(name..' ('..id..')',game.Players:children())
end)()
end)
set.MakeCommand('Stop Music',2,set.Prefix,{'stopmusic','musicoff'},{},0,function(plr,args)
for i, v in pairs(game.Workspace:children()) do if v:IsA("Sound") then v:Destroy() end end
end)
set.MakeCommand('Music List',2,set.Prefix,{'musiclist','listmusic','songs'},{},0,function(plr,args)
local listforclient={}
for i, v in pairs(set['MusicList']) do 
table.insert(listforclient,v.Name)
end
set.Remote(plr,'SetSetting','MusicList',listforclient)
set.Remote(plr,'Function','ListGui','Music List','MusicList')
end)