The exception is telling you that the PDB is NOT open.
A 'startup' command does NOT open any of the PDBs by default. When you first installed 12c and created a sample DB the installation process opened it. But then you likely shutdown the DB and pc - then you started the pc and the DB started automatically but ONLY the root/CDB gets started.
You connected as SYS to the root/CDB and set the container to the PDB but the PDB isn't open.
Multitenant has MANY differences from prior versions and you just ran into one of them. You have two options
1. create an AFTER STARTUP trigger that opens any PDBs you want opened automatically
2. open the PDBs manually - 'ALTER PLUGGABLE DATABASE ALL OPEN;'
I have replied MANY times in the multitenant forum answering this same basic question. For example see my replies in this thread
Now for the advice no one ever seems to 1) want to hear, 2) take seriously or 3) actually do.
RTFM - with 12c multitenant you will continue to have similar issues until you read and understand the multitenant architecture.
I suggest you first read chapters 17 and 18 of the Concepts doc about the new multitenant architecture of 12c.
There are 3 or 4 relevant threads in the multi-tenant forum that have bits and pieces of that info. So if you want you can read these threads, my replies and the links I provide to still other threads:
Thank you very much for your helpful and detailed response!