Hello @Magdy, welcome aboard to our community. I wonder if beside the message it gives you to come here, is telling you something else? I've seen in some user this happened when they have different email address in facebook and Fitbit, although In my case, this never gave me any issue. You can review ksarg's post for more information.
I recommend to force quit the app, just before you proceed log out from your Fitbit account:
1. Double tap the home button on your iOS device. Your open apps will show on your device's screen.
2. Close the Fitbit app by swiping the app image in an upward motion.
3. Reopen the Fitbit app and sign back in and try to link your facebook account.
You can try this too directly from the web-dashboard on your friends section if the issue persist.
Hope this helps and keep me posted how it goes.
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