A 15 day negative with a duo is encouraging but certainly not conclusive as stated by CurfewX. The p24 antigen part of the test must have been accurate, the antibody part of the test wasn't since it being in window period. However, for a solid result you need to repeat this test at 28 days.
What was your exposure, please?
This test is not intended to be used on day 15. Test at 28 days, if you actually had a risk, to know your status.