This from William Fitzsimmons: https://thechoice.blogs.nytimes.com/2009/09/11/harvarddean-part2/
Regardless of the information on Harvard’s website about SAT Subject Tests being optional, Harvard seems to value academic-based standardized tests, such as the SAT Subject Tests MORE highly than the SAT or ACT tests. So, my guess is the vast majority of applicants (your direct competition) will be submitting two SAT Subject Test scores along with their application. So, while it may not hurt your application if you do not have them, it demonstrates to Harvard that those students who have done well on SAT Subject tests (700+) will do well if admitted.
FWIW: My kid’s high school advised students taking non-AP courses in subjects such as Bio, Chem, Math and Physics to take the SAT Subject Test in June following the course, as their HS sufficiently prepared them to do well on Subject tests without any additional studying. You might check with your guidance counselor if the same applies for your HS. Even though SAT Subject Tests are optional, I would still submit them with your application if the cost of doing so is not a burden to your family.