Sounds like you have the device lock set on. This happens automatically if you set up fitbit pay.
From the manual:
To help keep your watch secure, turn on device lock in the Fitbit app, which prompts you to enter a personal 4-digit PIN code to unlock your watch. If you set up Fitbit Pay to make contactless payments from your watch, device lock is turned on automatically and you're required to set a code. If you don't use Fitbit Pay, device lock is optional. Turn on device lock or reset your PIN code in the Fitbit app: From the Fitbit app dashboard, tap or click the Account icon ( ) > Ionic tile > Device Lock
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