How to concatenate strings in delcared text (rich text)

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  1. What do you want to achieve? Keep it simple and clear!
    Concatenated strings in textlabel that uses rich text

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-- This is an example Lua code block

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You need to be far more specific.
To concatenate strings of any form in Lua you use the string concatenation operator, ..
What are you having trouble with?

if i use [[]] to use rich text, i cannot use “…”

Oh, those are called string literals, not rich text. It’s working fine for me.
print([[test ]]..[[test2]])

Unless you’re actually using three dots, which is the ellipsis operator for making variadic functions, not concatenating strings.

local player = game.Players.LocalPlayer

script.Parent.Text = [[<font size="30">player.Name</font>]]

Give this a go.
script.Parent.Text = [[<font size="30">]] .. player.Name .. [[</font>]]