。
You have to be ready for the unexpected.
Literal
Unforeseen-[classical-ぬ] occurrences [for-に] prepare-must-[polite-なりません].
予期せぬ is a classical-looking negative: せぬ is the old negative form of する, preserved in set phrases. Modern spoken Japanese would say 予期しない, but 予期せぬ remains the idiomatic way to say 'unforeseen' in this construction. 備える ('to prepare for, provide for, furnish with') takes an に-marked target — prepare FOR something. A tidy lesson on how classical forms survive in formal-written frames.