This is a known issue with all v3.0+ versions of the Fitbit app.
Two possible solutions: download the "Fit Notifications (for Fitbit)" app from the Play Store (developer is Smart Dinosaurs Entertainment) and set it up through their instructions.
If that doesn't work, uninstall the app and install v2.90 from a mirror online. (a. P. K. Mirror is a good place to find it)
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