Can I sacrifice my Reflection Token in the beginning phase?

To determine if you can use Sacrifice on a Reflection token during the Beginning Phase, please note that you cannot sacrifice a Reflection token, though you may otherwise be able to play the spell during the Beginning Phase if the target were valid.

1. Can you sacrifice a Reflection token?

No. The spell Sacrifice requires you to "kill a friendly [Mighty] unit." Rule 184.6 defines a Reflection token as a unit with "0 Might." Because a Reflection token does not meet the "Mighty" requirement, it cannot be chosen as the target for the cost of Sacrifice.

2. Can you play Sacrifice during the Beginning Phase?

Yes.

• Reaction Keyword: Because Sacrifice possesses the [Reaction] keyword, it can be played during Closed States on any player's turn per Rule 813.1.c.
• Beginning Phase Timing: Rule 813.2 specifies that the [Reaction] keyword is not restricted to Closed States and is inclusive of all other timings. When triggered abilities or game effects are added to the chain at the start of the Beginning Phase (Rule 315.2.a.1), the chain becomes a "Closed State." Consequently, you have the permission to play Sacrifice at any time you have priority, including while the chain is active during the Beginning Phase.

Summary

You cannot sacrifice a standard Reflection token because it does not meet the "Mighty" requirement for the spell. While you have the timing permission to play a [Reaction] spell like Sacrifice during the Beginning Phase, the Reflection token is an invalid target.

Note: I could not fully verify this ruling against the retrieved rules.