Hey, so I followed all the steps to install percona and it works perfectly until I simply restart the MySQL service then I always get this error : 
At this stage, I am totally depressed because I really don’t know what’s wrong.
Hi @Poli
I have tried to reproduce your issue but without success.
Could you please check syslog as it is recommended in the systemd output. Most probably pid file wasn’t deleted after last run or there is no permissions to create pidfile.
@Poli - are you looking to install Percona XtraDB Cluster or Percona Server for MySQL ?
Percona XtraDB Cluster needs to be installed on multiple nodes and specially “bootstrapped” on the first node. It can’t be just started as MySQL or Percona Server for MySQL
https://www.percona.com/doc/percona-xtradb-cluster/LATEST/install/index.html