password not recognized

[kalpana]

PC: 2015 Mac Mini, 1 T space, 8 gig memory
SW: 20.04 Ubuntu Server

Hi,
I am new to ubuntu server, learning as I have go along. Downloading and installing was no problem. Whole drive was used.
wifi, ethernet cable used. As I have learned more information, I could not figure out how to do it in the context of the server so I would reinstall the SW again. So I have installed the sever SW many times a and everything worked until lately.
I am planning to install LAMP so I can study databases. Everything has worked fine until I decided to change it again.
I wanted to add security so I reinstalled everything again and it would not recognize my password. I thought it may be my keyboard so I purchased another one but that was not it.
Maybe my HD is damaged but there is a check done before installing? Maybe someone has gotten into my system?
Honestly, I do not know what is wrong. PLEASE help!

Thanks in advance for your time and help.

PB