Learn English – “Composed of” Vs. “Consists of”

"Mandatory" means that the thing must be done due to some reason or rule.

These courses are mandatory.

That is, you must take the courses or else you fail.

Wearing helmets was made mandatory a few years ago.

It is a rule that you must wear helmets.

"Indispensable" is that the thing is so critical, useful, or important that you cannot conceivably throw the thing away.

The volunteers' help was indispensible.

The mission would have failed if the volunteers didn't help.

He made himself indispensible to the parish priest.

He was deemed a must-have person because of some good quality.

These great tools are so versatile that they are indispensable.

The tools are so good that you shouldn't be without them.

To sum up, these two words are not interchangeable - they have their own distinct meanings.

These two are actually very different.

That code should be debuggable.

This one is fine - this is the correct and natural way to say what you're trying to say.

That code should be able to debug.

This one made me laugh a little bit, because I'm pretty sure you didn't intend to write this. Plus I wish what you wrote actually existed. It would make things a lot easier for me.

So what you actually wrote here is that your code should have the ability to debug. In other words, you wrote some sort of intelligent AI that is able to debug other code. Man, that'd be amazing. Definitely not what you meant though.

What you were probably looking for was this

That code should be able to be debugged.

Now that is grammatically sound, however it is not ever said this way because the structure is needlessly complicated.

Stick with the first choice for practically all uses. They have equivalent meaning, but in my experience, the first choice is much more common.