Error occurred when you are using the Express edition. I tried to find the solution of using express edition but it failed to open my PDBs database and giving me error. So, i found one alternative as solution but it lost you data to fixed this problem. So please always track or monitor your database size for size 10 GB.

Error:

SQL> alter pluggable database all open;
alter pluggable database all open
*
ERROR at line 1:
ORA-12954: The request exceeds the maximum allowed database size of 12 GB

Alternative Solution:

Solution is to check all the datafiles size and check which PDB reached the size of 12gb then drop that pluggable database to continue using the Oracle Express edition.

1. Connect with CDB database as SYS user and check the size of all datafiles including PDBs

Col name for a25
select name, to_char(bytes / 1024/1024/1024, '99,990.0') || 'GB' datafile_size from v$datafile;

NAME                                               DATAFILE_SIZE
-------------------------------------------------- ----------------------------
C:\ORACLE\ORADATA\XE\SYSTEM01.DBF                        0.9GB
C:\ORACLE\ORADATA\XE\SYSAUX01.DBF                        0.6GB
C:\ORACLE\ORADATA\XE\UNDOTBS01.DBF                       0.1GB
C:\ORACLE\ORADATA\XE\PDBSEED\SYSTEM01.DBF                0.2GB
C:\ORACLE\ORADATA\XE\PDBSEED\SYSAUX01.DBF                0.4GB
C:\ORACLE\ORADATA\XE\USERS01.DBF                         0.0GB
C:\ORACLE\ORADATA\XE\PDBSEED\UNDOTBS01.DBF               0.1GB
C:\ORACLE\ORADATA\XE\XEPDB1\SYSTEM01.DBF                 0.3GB
C:\ORACLE\ORADATA\XE\XEPDB1\SYSAUX01.DBF                11.1GB
C:\ORACLE\ORADATA\XE\XEPDB1\UNDOTBS01.DBF                3.4GB
C:\ORACLE\ORADATA\XE\XEPDB1\USERS01.DBF                  0.0GB
C:\ORACLE\ORADATA\XE\PDB1\SYSTEM01.DBF                   0.3GB
C:\ORACLE\ORADATA\XE\PDB1\SYSAUX01.DBF                   0.4GB
C:\ORACLE\ORADATA\XE\PDB1\UNDOTBS01.DBF                  0.1GB
C:\ORACLE\ORADATA\XE\PDB1\USERS01.DBF                    0.0GB
C:\ORACLE\ORADATA\XE\PDB2\SYSTEM01.DBF                   0.3GB

2. If you found any PDB of 12 GB size then for continue the express edition you need to drop the PDBs. I found XEPDB1 is reached the limit thats why i am not able to open all other PDBS. You can take backup of all PDBs through RMAN or cold as you wish and drop the XEPDB1 database.

SQL> DROP PLUGGABLE DATABASE XEPDB1 INCLUDING DATAFILES;
Pluggable database dropped.

3. Try to open the database again.

SQL> alter pluggable database all open;
Pluggable database altered.