Will US graduate schools accept me retaking a GRE subject test in the spring?
A poor test score is not disqualifying if the rest of your application is strong.

However, an April test date will not work for admission next fall. The reason for this is that the US admissions cycle requires acceptance decisions around mid-April. Since the test is in mid-April, and scores won't be available until mid-May, they would have to accept you "blind," without knowing what your scores would be. So you're essentially "stuck" with the scores you have, at least for admission for the upcoming fall semester.