How do I resolve this "ORA-01109: database not open" error?

As the error states - the database is not open - it was previously shut down, and someone left it in the middle of the startup process. They may either be intentional, or unintentional (i.e., it was supposed to be open, but failed to do so).

Assuming that's nothing wrong with the database itself, you could open it with a simple statement:(Since the question is asked specifically in the context of SQLPlus, kindly remember to put a statement terminator(Semicolon) at the end mandatorily, otherwise, it will result in an error.)

ALTER DATABASE OPEN;

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.