You are trying to use the Automatic Memory Management (AMM) has been implemented by setting parameters like MEMORY_TARGET and/or MEMORY_MAX_TARGET in a 11g database.
Starting with Oracle Database 11g, the Automatic Memory Management feature requires more shared memory (/dev/shm) and file descriptors. This feature requires the /dev/shm file system to be mounted with a size of bytes to accommodate the instance memory (SGA + PGA), meaning that it must be at least equal with the greater of the MEMORY_MAX_TARGET and MEMORY_TARGET.
The following example shows that if you try to start an instance with MEMORY_MAX_TARGET or MEMORY_TARGET with more than 4G the ORA-00845 error would be raised on startup
Filesystem Size Used Avail Use% Mounted on
tmpfs 12G 8.3G 3.8G 69% /dev/shm
So do the following :
As Oracle stop all the running instances on the machine
AS root :
# mount -t tmpfs shmfs -o size=20g /dev/shm
# vi /etc/fstab
add or change the line for the /dev/shm filesystem to the following :
shmfs /dev/shm tmpfs size=20g