Can you play a spell before the beginning phase?

No, you cannot play a spell before the Beginning Phase.

According to the game structure defined in Rule 315, the turn begins with the Awaken Phase, followed immediately by the Beginning Phase. There is no phase or priority window that exists before the Awaken Phase begins.

• Awaken Phase (Rule 315.1): The turn player readies all game objects they control.
• Beginning Phase (Rule 315.2): Game effects take place at the start of this phase.

Spells can generally only be played during an Open State (typically during your Action Phase) or in response to something on the chain (if the spell has the [Reaction] keyword). Since the game has not yet reached a state where players have priority to play spells before the Awaken Phase, no spells can be cast at that time.