Is it permissible to put on tefillin on Yom Kippur?

Shalom,

On Shabbat and holidays (and Yom Kippur is considered a holiday), it is forbidden to put on tefillin. The reason for this is because Shabbat itself is a sign between Hashem and Israel, as it is written (Exodus 31:13), "For it is a sign between Me and you." If one were to put tefillin, which is also a sign, as it is written (Exodus 13:9), "And it shall be for a sign upon your hand," it would be a disrespect to the sign of Shabbat. This is also the reason that tefillin is not worn on holidays.

Sources: Shulchan Aruch Orach Chaim (siman 30, se'if 1) and Mishnah Berurah (s.k. 3-4).

Wishing you success,

Hillel Meirs