If tables of a SCHEMA for example SCHEMA_ABC are implemented to create the modelling views, then the following SQL statement must be performed before starting any such modelling views.

GRANT SELECT ON SCHEMA SCHEMA_ABC TO _SYS_REPO with the option of GRANT
Consider _SYS_REPO as “the activation guy”. It takes your models and makes the required runtime OBJECTS from them. Thus user _SYS_REPO requires the allowance to choose YOUR tables/views. (If _SYS_REPO user is unable to decide on the tables added in the from-clause of the view-definition, it will FAIL to describe that view). If different customers REQUIRE to choose this view (It is quite obvious this is always the scenario, else the views would not make any sense), then _SYS_REPO ought to have the extra allowance to grant the selection further (with the grant option).

After all your models that access data in your schemas has been activated, _SYS_REPO wants to OFFER you and other users read access to the activated models.