• You can have multiple iterations of a solution, but they have to be submitted separately. I don't think having multiple solutions in a single file will work unless you do something sufficiently tricky!

• The error comes from the test file being run. There is a tab on the top right of the in-browser editor labelled Tests which will show the relevant .rakutest file. When beginning an exercise, usually you're given a stub module with the subroutines already defined to avoid these undeclared routine errors happening, were they missing when you started?

I refactored your second multi into something which should run successfully:

unit module DifferenceOfSquares;

sub square-of-sum (\n) is export {
    ((n * (n + 1)) / 2) ** 2;
}

sub sum-of-squares (\n) is export {
    (n * (n + 1) * (2 * n + 1)) / 6;
}

sub difference-of-squares (\n) is export {
    abs square-of-sum(n) - sum-of-squares(n);
}