Does anyone know why it is erroring me every time I try to call this function?

If you tabbed everything correctly, the script would look like this:

local function meteorsDisaster()
    local explosionPart = script.Prefabs.ExplosionPart
    local gameRoundLength = 10
    local gameInterMissonLength = 10
    local roundStartTime 

    local function wallOfDoomDisaster()
        local wallOfDoom = script.Prefabs.WallOfDoom
        wallOfDoom.Parent = game.Workspace
    end

    local function initialize()
        roundStartTime = tick()
    end

    local function createExplosionPartCopy()
        local explosionPartCopy = explosionPart:Clone()
        explosionPartCopy.Parent = game.Workspace
        local xPosition = math.random(-64, 79)
        local zPosition = math.random(-45, 90)
        explosionPartCopy.Position = Vector3.new(xPosition, 100, zPosition)
    end

    while true do
        initialize()

        repeat
            local currentTime = tick
            local timeSinceGameStarted = currentTime - roundStartTime
            createExplosionPartCopy()
            wait(.5)
        until timeSinceGameStarted > gameRoundLength

        wait(gameInterMissonLength)

        wallOfDoomDisaster()
    end
end

wallOfDoomDisaster()

Do you see the problem? You declared wallOfDoomDisaster as a local variable to the meteorsDisaster function. Presumably, you never finished it and just put an 'end' at the bottom of the script when the error showed up. Always indenting properly is encouraged to help find errors like this.

You need to move one of the final ends (ex line 39) to line 2. Note that the final call to wallOfDoomDisaster won't run because the while true do loop will never exit, so remove it (or, for testing purposes, move it to before the while true do loop).