That method returns a string, not a number. Please read the docs. Convert the string to a number.

How do I return the string when the .toFixed method is only work on number?

You're returning a string, not a number.

Example gives result of 100.00 (rounding to 2 decimals), but test cases give result of 100, and 421... Frustrating

It's frustrating when bocce is explained, then the number of balls for red is different than black, and not 3 each. It's not clear in bocce rules what to do when there are 2 red balls and 5 black; I compare 2 closest for each, but that doesn't seem to be correct. I like the bocce scoring concept, but testing thows in very odd data.

Maybe be a repeat of value-counting katas, but more practice with a slight variation lets you see how to apply different techniques to solving a kata.

Maybe add - Challenge - See if you an you solve this in one line.
Gets beginners thinking about simplifying code

I also got 'Execution Timed Out'

A few semantics suggestions:
*delete the line 'The value is supposed to be an integer' and change the top line to be 'Your job is to return the amount of metal (an integer) needed...'
*change 'The coaches take 5 pieces of metal to make(for each) 'to 'The coaches take 5 pieces of metal each'
*I'd also suggest your two examples give different results - maybe something like 'D________' which is 48

Ok, will correct that

In your description - if a = 4... you say '3' should be returned, but really you don't want a string returned but an int; it might be better to say sum_diff(4,2,3) should return 3
Nice beginner kata

A few suggestions:
*This is first in a series, not serie.
*It would be nice to explain what a DNI is at the beginning.
*In your description you say a'This relation is that the

rest of the division of the large number by 23 is ..." in English, we usually say the remainder when dividing by 23 correlates to the letter in the following table"

*I'd suggest modifying your basic function def from def dni_solver(dni): return dni to def dni_solver(str); return dni because you are not feeding the function dni and expecting to return dni also
*In the description you say that the task is to determine the letter of the DNI number; so why not just provide a number and return a letter only? or is the task to provide the completed DNI
*Those comments aside, I did enjoy doing kata where one learns a bit about another culture.