I think that i found the solution:

when i started the ambari installation, I stupidly change the default password which is bigdata to an other

So when the server goes to start it retrieve the password which is inside /etc/ambari-server/conf/password.dat in order to connect to Mysql which is obviously not the same.

I forced the change of the password in the path: /etc/ambari-server/conf/password.dat and then it works fine

thanks @Jay Kumar SenSharma