Why do I have to use 'was' in this sentence?
Maybe it is that way because in german the sentence means something like "all those things (alles) that (was) you have...". In that case the "that" (was) is requiered.
Same mistake, same question? Why?
English has reduced relative clauses—you can remove the "that" or "who" or "whom" and it still works. German, among most other languages, doesn't do that.
I think it's 'etwas' shortened once again, but not sure.
Dienstleistung is also a correct translation for service. Why is it not accepted?
Dienstleistung is feminine. If you put der Dienstleistung, it would be wrong. Otherwise I would assume die Dienstleistung would be correct.