As a public institution, the UW is dedicated to first-time degree-seeking students. Coupled with high demand for Math degrees amongst Undergraduates, unfortunately means math degrees are not available for Post-bac students through the UW Seattle.
If you are are seeking a math degree in order to get a teaching endorsement you do not need a math degree for the endorsement- only the coursework. See the Secondary Teaching Endorsement page for more information.
For students seeking math coursework for other reasons (other jobs, graduate programs) should either apply for programs geared towards them (i.e. graduate programs or specialized post-bac programs) or consider what they actually need to achieve their goals. In a vast majority of cases a math degree is not the only path. Taking classes as a non-matriculated student is an option, as is diligent self-study.