Community Moderator Alumni are previous members of the Moderation Team, which ensures conversations are friendly, factual, and on-topic. Moderators are here to answer questions, escalate bugs, and make sure your voice is heard by the larger Fitbit team. Learn more
Hello @Chih-Yuan, have a warm welcome to the Fitbit Community, it's great to have you on board. 
This seems like a very odd situation, have you restarted your phone already? If so, I would like to suggest you to disable mobile data on your phone, then wait 15 seconds and enable it back and try to sync your tracker while WiFi is disabled.
I hope this can be helpful, give it a try and keep me posted in case you need anything else. 
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Community Moderator Alumni are previous members of the Moderation Team, which ensures conversations are friendly, factual, and on-topic. Moderators are here to answer questions, escalate bugs, and make sure your voice is heard by the larger Fitbit team. Learn more
Hello @Chih-Yuan, I hope you're doing well, thanks for taking the time to reply and let me know your issue has now been resolved, I'm very glad.
In case you have some spare time, I would like to invite you to visit our Discussions board where you will find great tips and encouragement from other members and if there's anything else I can do for you, please keep me posted.
Happy stepping. 
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