The biggest problem is the inability to decide what to do about priorities.
Literal
Priority [object-を] how should [question-か] decide-cannot [nominalizer-の] [subject-が], greatest problem is.
An embedded question (どうすべきか, 'what should be done') feeds into 決定できない ('cannot decide'), which is nominalized by の and marked as the subject by が. Formal written style throughout.