Registering with geodatabase is an important task in working with geospatial data in different database situations.
When you want to register a dataset with the geodatabase it creates or uses a ObjectID inside the table.
Unfortunately neither in the documentation nor in a warning/error message it is described, that the SHAPE column needs to be "nullable" to register a dataset with the geodatabase.
Please update the online/tool help and rewrite the error message to
SHAPE column must not have a NOT NULL constraint
instead of:
The column must not have a NOT NULL constraint
Or in a general way:
[COLUMNAME] column must not have a NOT NULL constraint
instead of
The column must not have a NOT NULL constraint