It's not your pairing: Since September 30, 2026, no data is transferred from Fitbit to Fddb. Google has discontinued the previous Fitbit interface; Fitbit is now called Google Health. A new pairing with Fitbit is no longer possible — neither in the Fddb app nor on the Fddb website.
You can still import your data from Google Health into the Fddb app, though: on iOS via Apple Health, on Android via Health Connect.
To learn how to switch and what happens to your previously imported data, see The Fitbit connection has been discontinued. What can I do?