(more complete answer: "it's an empty string")

;)

java.lang.StringIndexOutOfBoundsException: String index out of range: 0      <<< info
at java.base/java.lang.StringLatin1.charAt(StringLatin1.java:47)             <<< noise
at java.base/java.lang.String.charAt(String.java:693)                        <<< info
at Kata.encryptThis(Kata.java:13)                                            <<< info
...                                                                          <<< lot of noise

fourth: it's in your code, line 13
third: it's a charAt method
first: index out of range, at index 0

CCL: what's the string you're trying to index?

414 Java completions

Not an issue. Your code is the source of the error.