It would be better if you posted a DDL statements you used to create the tables. Firstly, others would be able to test your example (see SSCCE), and secondly, in this case the existence (or the lack of) of primary keys and foreign constraints is crucial, and we don't see it from the descriptions.
Nevertheless, the error message mentions non key-preserved table. Thomas Kyte has probably the simplest definition what key-preserved view means:
Thomas Kyte wrote:Key preserved means the row from the base table will appear AT MOST ONCE in the output view on that table
I haven't ever tried to insert into a join, but I'd say that it would only be possible if the tables in the join were in a 1:1 relationship (and declared by existing constraints as such), which is quite rare.