Hello,

(Almost) all addresses in the Brussels-Capital Region are already in the database.  StreetComplete is a nice app but it does not know about the situation in every place.
We advise you to turn off the quests about missing addresses when you map in Brussels, via the Settings.  Alternatively, if it asks you something like this, please use the option that says that this building does not have its own housenumber.  But please never duplicate existing values, it just corrupts the database.

Thanks in advance and happy mapping.