Linux is case sensitive and by far the most common fault is not typing precisely what you set when installing the system.
Another common fault is, you had the "Caps Lock" on when typing your password, so try it like that as well; just in case!
Fujitsu Lifebook P727 laptop review
Microsoft Paint set to die after 32 years
Mac power user tips and hidden tricks
Comment désactiver la saisie intuitive et paramétrer votre clavier ?