This is an SQL Developer question, so would be better in that forum.
sometimes people forget to truncate or delete the migration repository from the connection.. even u reinstall the software, u must truncate or delete the repository
just right click on the connection and choose delete mgr rpstory
for me, it works
there are 2 steps to follow.
1. create user (in oracle database not through grant command)
2. grant specific permissions including CREATE MATERIALIZED VIEW