Expected 'do' when parsing while loop, got 'X'

What the message means

ServerScriptService.Spawner:3: Expected 'do' when parsing while loop, got 'then'

Other forms of the same message:

• Expected 'do' when parsing for loop, got 'print'
• Expected 'do' when parsing for loop, got 'then'
• Expected ',' when parsing index range, got 'to'

It is a syntax error, so the whole script does not run. The line in Output is where Luau found the wrong word, which is the next line when do is missing at the end of the loop line.

do is for loops and then is for if. The reverse mistake, do on an if, gives Expected 'then' when parsing if statement.

Causes and fixes

Three things cause this error in practice. Each one below has the smallest code that triggers it, then the fix.

• then written instead of do
• do is missing at the end of a for loop
• for i = 1 to 10 (a range written with to)

then written instead of do

while has a condition like if does, so it is easy to end it with then.

local enemiesLeft = 3

while enemiesLeft > 0 then
	enemiesLeft -= 1
end

End every while and for line with do.

local enemiesLeft = 3

while enemiesLeft > 0 do
	enemiesLeft -= 1
	task.wait(1)
end
print("wave cleared")

A while loop that runs for a long time needs a task.wait() inside it, or Studio stops the script with Script timeout: exhausted allowed execution time.

do is missing at the end of a for loop

Luau reads the loop header and then the next word. Without do, that word is the first line of the loop's body.

local Players = game:GetService("Players")

for _, player in Players:GetPlayers()
	print(player.Name)
end

Add do at the end of the for line.

local Players = game:GetService("Players")

for _, player in Players:GetPlayers() do
	print(player.Name)
end

for i = 1 to 10 (a range written with to)

Some languages write a counting loop with to. Luau uses a comma: for i = 1, 10 do. With to, Luau reports Expected ',' when parsing index range, got 'to'.

for i = 1 to 10 do
	print(i)
end

Write the start, the end and an optional step, separated by commas.

for i = 10, 1, -1 do
	print(i)
	task.wait(1)
end
print("Go!")

How to find the line that causes it

1. A syntax error stops the whole script before its first line runs, so nothing in that script works, not just the broken line.
2. Open the script. The Script Editor underlines the problem in red; hover over the underline to read the message.
3. Open Script Analysis (View tab) to see every syntax error in every script, with its line number. Double-click one to jump to it.
4. The line number in the message is where Luau gave up, which is often below the real mistake. The (to close ... at line N) part points at the line where the unclosed bracket or block starts.

Questions

Sources and further reading

• Luau: syntax. generalized iteration
• Roblox Creator Docs: control structures. while, for and repeat loops

How the code was checked: every block marked Fixed passes Luau's strict type checker against Roblox's API definitions (luau-lsp, 5 November 2026), and every block marked Breaks compiles only to fail with the message on this page. The message wording was checked by compiling each broken block on this page with luau-compile, which uses the same Luau parser as Roblox Studio, on 5 November 2026. A type check catches misspelt APIs and wrong types, not game logic, so playtest in Studio before you publish.