For a solar eclipse to happen, there must be a new moon. This is because the Moon must be positioned between the Earth and the Sun for it to block (partially or completely) the Sun's light.
If the Moon is in any other phase — such as a full moon, first quarter, or last quarter — it will not be positioned between the Earth and the Sun, and therefore, a solar eclipse cannot occur. The alignment of the Sun, Moon, and Earth required for a solar eclipse is only possible during the new moon phase.