Can I use a Reaction at the beginning phase before the Awaken Phase?

No, you cannot use a Reaction at the beginning phase before the Awaken Phase.

According to RiftJudge FAQ #10793 and RiftJudge FAQ #9538, you do not receive priority simply because a turn or phase has begun. Priority is only granted to you when your opponent performs an action that creates a chain (such as playing a spell or triggering an ability that uses the chain) or during a showdown.

Furthermore, RiftJudge FAQ #5566 clarifies that you cannot activate reactions during the Beginning phase without a trigger creating a chain. Reactions require a trigger to open a chain; they cannot be activated freely at the start of a phase.