Is "What do I have to add" wrong?
Nope, not wrong at all. "What do I have to add?" is perfectly idiomatic English.
Well, try reporting this and wait for an answer from the team. They may accept it! =)
I thought that "devo" was "must". In an earlier exercise: "O motorista deve observar as regras..."
I'm sure you gonna have a tough time in this section. They keep mixing them up and dever in Portuguese can also mean "should", depending on the context.
is "what should I increase?" wrong?
It's ok, but:
acrescentar / adicionar = add
aumentar / incrementar = increase
Why is "What do I need to add wrong?"
It should be accepted as correct.
Do people really use acrescentar? It's sounds to me like using increase "instead" of "add" in english. And even if they do, are there other words that are used instead of acrescentar?
Yes. It also is used to say a few more words.
I must add/say = eu devo acrescentar
What's the function of Devo here?
It is a conjugation for the verb "dever". Most of the time, it translates the verb "must", but also "should" when it has a sense of obligation.
For "I must", would you use "eu devo", and for "I should", would you use "eu deveria"? Do I have that right?
Yes, it is also right.