Your problem is in line 3 of your code:

Note: no empty arrays will be given.

But you're generating an empty array.

And then there is another problem:

If there is a tie for most frequent number, return the largest number among them.

Your code returns the first value, not the largest one.

The problem is when you populate clean_str

I've renamed the function, there was a typo in its name, change it to pangram and click reset.

p panagram?("The quick brown fox jumps over the lazy dog") == true

It has both T and t, your code is still not case insensitive.

Changed the error messages, but your code is wrong, it's not case insensitive.

That needs Ruby >= 2.7