Caesar Cypher issue

So today i was coding a Lua caesar cypher based off of my Java one i created a while back, problem is im a terrable Lua programer. I made what i have so far by looking at other programs to learn how Lua booleans and stuff work. What i have so far works… ish.

Spoiler

print("Welcome to the monkey cypher")
print("Please enter encryption key")
key = read()
function encrypt(data)
i = 1
enc = ""
repeat
enc = enc .. string.char(data:byte(i) + key)
i = i+1
until data:byte(i) == nil
return enc
end
print("Enter text to encrypt: ")
str = read()
str = string.upper(str)
print("Original message: " .. str)
str = encrypt(str)
print("Encrypted message: " .. str)
print("")
print("To decrypt use the inverse key you just used.")
print("Ex. If i used 6 to encrypt the message i would use -6 to decrypt it")

Basicaly it converts every letter to its numerical value, adds the key, then converts it back to its letter value. The issue is that lower case, upper case, and symbols are all in the same numerical system. So after Z you don't get A you get some symbol. Right now my caesar works if it doesn't have to convert anything past Z. A + 6 = G, but Z + 6 = ?.
What i need to do is have a loop that says "If enc + key > 'Z' then enc = enc - 26" but im not sure how to do that in Lua. Any ideas?

You could do % as modulus?
For example the time is 23h . And you want to know the time 4 hours later:
(23+4) % 24 = 3

Also I would not recommend using the ANSI- alphabet, because some characters are unprintable and result in ?

Its not that they result in a ? because they are unprintible. Its that symbols are numericaly after capitol letters.

If the value of Z + 6 is more than Z, take away Z. Likewise from the other direction, but instead add Z.