SIMPLE ANSWER: You most likely declared UPROPERTY() within your header file as private or forgot to make it public since classes members are private by default (hot tip). This I found to be the main issue for most people getting this compiler error.

SIMPLE SOLUTION: Declare public: before writing out your UPROPERTY() or use the "AllowPrivateAccess" property such shown in the example below.

Ex. UPROPERTY(VisibleAnywhere, BlueprintReadOnly, meta=(AllowPrivateAccess = true))

I hope this helps. Give thumps up if it did.