loadstring loads Lua code from a string into a function than you can call (to execute the loaded code). Looking at the loadstring error message, it does not appear that you're putting code into that, but rather a string of text, not code.
You have two options:
Either change line 65 to
memo = guiCreateMemo(241, 24, 450, 437, "Bem vindo/a ao RPG City Reallife. Use o menu á esquerda para navegar pela documentação do servidor.\n\nRegras do Servidor:\n\n"..rules, false, window)
and remove line 28 through 34 (the safer option, since you remove the ability for arbitrary code execution in your script)
Or the second option, is to change the rules.txt file to contain valid code which this script expects. It appears to expect a table named theRules with a string of rules under the key "English", so your rules.txt should be
local theRules = {
["English"] = "Art 1° - Não praticar Power Gaming: Mínimo (10H) Máximo(24H ..."
}
and it should load correctly. However, this is less safe, since it can allow someone with access to editing rules.txt to insert malicious code into the file, which is then executed on through clients' Lua interpreters.