How can I fix this sound script?

I would recommend using a Local Script to fix your problem.

There might be a problem with the script's ability to read the sound's TimeLength property. To fix this, use a local script.

"But where would I put the Local Script?"

I would suggest putting the Local Script in StarterPack. You could also put it in StarterGui or even ReplicatedFirst. However, StarterPack will be the one I use in my example.

"But what would I have to change?"

Not much actually. You would have to move the script to StarterPack, and change the script into a Local Script, along with a few other changes.

"Will it be hard?"

Nah.

• First,

Change the script into a Local Script and put it into StarterPack.

-- LocalScript in StarterPack
local songs = {'DeathOfABachelor', 'DontThreatenMeWithAGoodTime', 'Focus', 'IntoYou', 'MrsPotatoHead', 'MyHouse', 'No', 'PityParty', 'Radioactive', 'WorkFromHome', 'iWriteSinsNotTragedies', 'Work', 'StressedOut'} --Lol this is a lot of songs :P

for i=1,#songs do
    local sound = script.OST:findFirstChild(songs[i])
    sound:Play()
    wait(sound.TimeLength)
end

• Second,

Now we have access to some sound properties that would otherwise be unobtainable. We'll be waiting for the event Ended to fire from the sound in place of line 7 in the script above.

-- LocalScript in StarterPack
local songs = {'DeathOfABachelor', 'DontThreatenMeWithAGoodTime', 'Focus', 'IntoYou', 'MrsPotatoHead', 'MyHouse', 'No', 'PityParty', 'Radioactive', 'WorkFromHome', 'iWriteSinsNotTragedies', 'Work', 'StressedOut'} --Lol this is a lot of songs :P

for i=1,#songs do
    local sound = script.OST:findFirstChild(songs[i])
    sound:Play()
    sound.Ended:wait()
end

• Finally,

You have to wait for OST to load using WaitForChild, like so,

-- LocalScript in StarterPack
local songs = {'DeathOfABachelor', 'DontThreatenMeWithAGoodTime', 'Focus', 'IntoYou', 'MrsPotatoHead', 'MyHouse', 'No', 'PityParty', 'Radioactive', 'WorkFromHome', 'iWriteSinsNotTragedies', 'Work', 'StressedOut'} --Lol this is a lot of songs :P

for i=1,#songs do
    local sound = script:WaitForChild("OST"):FindFirstChild(songs[i])-- Wait For OST to load
    sound:Play()
    sound.Ended:wait()
end

I hope I helped. If I did, remember to accept my answer. It helps a lot!

Good Luck!