You must have "Coach" or "Club Director" access to upload power rankings.
The reason you are seeing this message is that an opponent has already reported their score for the date you have played, and their scores do not match yours.
All scores entered by an opponent that is being used for Power Rankings can be found by clicking on the Power Rankings Export link under each team's result page.
If a score was reported incorrectly, please reach out to a staff member of that club to request a correction.