Welcome to the forum @Hadi_Asgari
Does this mean it’s a new backup, and you have no others on Dropbox?
If you have a working Dropbox backup (or an export of it), try old AuthID.
Duplicati - Dropbox Backup - Short lived token expires supported new Dropbox need two days ago.
Possibly there’s a problem with the with the new code. There’s a parallel situation in GitHub issues:
[Dropbox oauth] expired_access_token error from Dropbox short-lived access tokens #4667