Why do my scripts act differently online?

I am used to testing things in Solo Play on studio but when I went to test Online, it doesn't work anymore and I get a bunch of errors that I fixed using "wait(5)"

One error is related to the fact that the StarterGui loads after the Backpack.

Not only that but a bunch of scripts unrelated to the GUIs only have bugged and don't even show a error output.

Is there a simple fix or do I need to go through my scripts again?

PS: Since scripts running in Solo Play are local, does changing all the scripts into Local Script works?

Your problems seem to be ones of timing. When using Play Solo (and to a lesser extent Start Server > Start Player), everything pretty much loads instantly. You don't have to check if something you put there exists: it will already have loaded.

Play Solo loads so fast, even, that PlayerAdded events may not have a chance to connect before the Player has joined.

I don't know your scripts, but I suggest going through them and figuring out what they are assuming to exist. If you're not absolutely sure that what they're assuming exists actually does (you can only be absolutely sure if the assumption in question is a direct ancestor of the script), use the WaitForChild method of all Instances (that aren't in nil) to, well, wait for what you assume exists to exist.