I'll probably be working on the report all day tomorrow.

Literal

Tomorrow [topic-は] all-day report [object-を] create-ongoing [probably-でしょう].

レポートを作成しているでしょう uses ~ている (ongoing action) + でしょう (probability), predicting a future ongoing state — 'I will probably be in the process of writing.' 作成する is a formal suru-verb for 'to create/compose.' 一日中 marks the duration.