Since the department head's schedule won't work, the date for the drinking party needs to be rescheduled.
Literal
Department-head [possessive-の] convenience [subject-が] bad became [reason-ので], drinking-party [possessive-の] schedule [topic-は] start-over is [emphasis-ね].
都合が悪くなった means 'the schedule became inconvenient.' 仕切り直し means 'starting over/rescheduling.' 飲み会 is 'drinking party' — a core Japanese workplace social concept.