I faced with that error message on 12.1.0.2.0 EE version on RHEL6

Here are the error messages

SQL> startup nomount;
ORA-27126: unable to lock shared memory segment in core
Linux-x86_64 Error: 12: Cannot allocate memory
Additional information: 3894
Additional information: 2129923
Additional information: 6090752

If you face same issue first check your ram size and your sga_max_size parameter. If your sga_max_size higher than your ram, resize that parameter first.
If your ram&sga_max_size seems okey than check below settings on your system

1. Add or amend the following lines in the “/etc/sysctl.conf” file.

fs.file-max = 6815744
kernel.sem = 250 32000 100 128
kernel.shmmni = 4096
kernel.shmall = 1073741824
kernel.shmmax = 4398046511104
net.core.rmem_default = 262144
net.core.rmem_max = 4194304
net.core.wmem_default = 262144
net.core.wmem_max = 1048576
fs.aio-max-nr = 1048576
net.ipv4.ip_local_port_range = 9000 65500

2. Run the following command to change the current kernel parameters.

/sbin/sysctl -p

3. Add the following lines to the “/etc/security/limits.conf” file.

oracle soft nofile 1024
oracle hard nofile 65536
oracle soft nproc 16384
oracle hard nproc 16384
oracle soft stack 10240
oracle hard stack 32768

4. Amend the “/etc/security/limits.d/90-nproc.conf” file as described below. See MOS Note [ID 1487773.1]

# Change this
* soft nproc 1024

# To this
* – nproc 16384

5. Set secure Linux to permissive by editing the “/etc/selinux/config” file, making sure the SELINUX flag is set as follows.

SELINUX=permissive
Once the change is complete, restart the server or run the following command.

# setenforce Permissive

If all are okay the issue related with LOCK_SGA=TRUE parameter. If that parameter has been set on your init file, just disable it or set as FALSE

Reference:
NOTE:827656.1- LOCK_SGA=TRUE Causes An ORA-27126 During Database Startup On AIX
BUG:8439698 – STARTUP WITH LOCK_SGA=TRUE FAILS WITH ORA-27126: UNABLE TO LOCK SHARED MEMORY SE
NOTE:361323.1 – HugePages on Linux: What It Is… and What It Is Not…

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