I got a same problem. Below is how I solved the problem. I am working on an oracle database 12c pluggable database(pdb) on a windows 10.
-- using sqlplus to login as sysdba from a terminal; Below is an example:
sqlplus sys/@orclpdb as sysdba
-- First check your database status;
SQL> select name, open_mode from v$pdbs;
-- It shows the database is mounted in my case. If yours is not mounted, you should mount the database first.
-- Next open the database for read/write
SQL> ALTER PLUGGABLE DATABASE OPEN; (or ALTER PLUGGABLE DATABASE YOURDATABASENAME OPEN;)
-- Check the status again.
SQL> select name, open_mode from v$pdbs;
-- Now your dababase should be open for read/write and you should be able to create schemas, etc.