Is it impossible to get a positive pregnancy test at 6 dpo??

Yes it is: Even assuming you knew exactly when ovulation or fertilization happened, a human embryo cannot start implanting until between 6-7 days after fertilization and cannot begin making detectable hCG until a few days later. thus, no pregnancy test can work that early.

Not impossible: It would be unlikely. The pregnancy test is positive after the embryo implants, and home pregnancy tests are positive usually 12 to 19 days after ovulation. Laboratory measures of hCG can be detected between 9 and 16 days after ovulation. If your pregnancy test is positive, possibly your estimate of your ovulation day is off.