The gnat compiler will recognize the word "const" as a bad abbreviation of the keyword "constant".
In the C/C++ family language (including Java, JavaScript, PHP, and some others) you mark a variable as a constant using the "const" keyword. It is not rare that someone will use the wrong keyword in Ada when used to also write C like code.
This error reminds you of the error.
max: constant max_length := 123;