migrating mySql DB to another partition resulted in mySql cant start with an exit code

We tried to move current mySql DB out of Linux system
partition by following the instructions in the links below
but mySql gave an error after migration:

https://www.digitalocean.com/community/tutorials/how-to-change-a-mysql-data-directory-to-a-new-location-on-centos-7

& how do I change the location of the data directory? (Doc ID 1023178.1)
https://www.digitalocean.com/community/tutorials/how-to-move-a-mysql-data-directory-to-a-new-location-on-ubuntu-16-04#step-3-%E2%80%94-configuring-apparmor-access-control-rules

# systemctl start mysqld
Job for mysqld.service failed because the control process exited with error code. See "systemctl status mysqld.service" and "journalctl -xe" for details.

DBA colleague told me there's not much of a clue from journalctl -xe
We're on RHEL 7.   Have also tried to create soft links to new partition
but it did not work.

What's missed?
Hopefull don't have to export all data out, recreate DB & re-import:
we don't want this option