Is it ensure, insure, or assure?
incompatible in·com·pat·i·ble (ĭn'kəm-pāt'ə-bəl)
Incapable of associating or blending or of being associated or blended because of disharmony, incongruity, or antagonism.
Producing an undesirable effect when used in combination with a particular substance, as a medicine in combination with alcohol.
Not suitable for combination or administration because of immunological differences, as blood types.