Can not install oracle apex on GNU/Linux (db 12c)
Check the status of pluggable database.

SQL&gt; select name,open_mode from v$pdbs;

NAME          OPEN_MODE
------------------------------ ----------
PDB$SEED         MOUNTED
PDBORCL          MIGRATE
PDBORCL2         MIGRATE
PDBORCL1         MIGRATE

SQL&gt; startup;

ORA-01081: cannot start already-running ORACLE - shut it down first

SQL&gt; shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.


Check the status of pluggable database.

SQL&gt; STARTUP;
ORACLE instance started.

Total System Global Area  788529152 bytes
Fixed Size      2929352 bytes
Variable Size    541068600 bytes
Database Buffers   239075328 bytes
Redo Buffers      5455872 bytes
Database mounted.
Database opened.
SQL&gt; select name,open_mode from v$pdbs;

NAME          OPEN_MODE
------------------------------ ----------
PDB$SEED         MOUNTED
PDBORCL          MOUNTED
PDBORCL2         MOUNTED
PDBORCL1         MOUNTED


We need to start PDB$SEED pluggable database in UPGRADE state for that 

SQL&gt; SHUTDOWN IMMEDIATE;


Database closed.
Database dismounted.
ORACLE instance shut down.

SQL&gt; STARTUP UPGRADE;

ORACLE instance started.

Total System Global Area  788529152 bytes
Fixed Size      2929352 bytes
Variable Size    541068600 bytes
Database Buffers   239075328 bytes
Redo Buffers      5455872 bytes
Database mounted.
Database opened.

SQL&gt; ALTER PLUGGABLE DATABASE ALL OPEN UPGRADE;
Pluggable database altered.

SQL&gt; select name,open_mode from v$pdbs;

NAME          OPEN_MODE
------------------------------ ----------
PDB$SEED         MIGRATE
PDBORCL          MIGRATE
PDBORCL2         MIGRATE
PDBORCL1         MIGRATE

SQL&gt; exit


try again 
it will solve the issue