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;