Are you receiving the "ORA-00845: MEMORY_TARGET not supported on this system" error while issuing the startup command for an Oracle database ? This error message is actually misleading because even though the MEMORY_TARGET initialization parameter (automatic memory management) may be supported on the server, the server may not have enough memory to startup one more database instance.
So please review the already started Oracle database instances and shutdown one or more less priority Oracle database instances.
1) Login to the Oracle database server
2) ps -ef | grep -i "pmon" (To find the running database instance names)
3) Then set the environment of one of these running database instances.
4) sqlplus / as sysdba
5) shutdown immediate; (After getting approval that it is OK to shutdown the instance.)
7) Now set the environment of the database instance that you wish to startup.
8) sqlplus / as sysdba
9) startup (This time you should not receive the ORA-00845 error.)
A long term solution is to assign more memory to the database server so that you do not have to shutdown some database instances to startup other database instances.