Cannot login. Unknown Username

I am a complete WordPress novice and need some assistance. I have received a copy of my company’s website and was told to get it online. I have successfully completed that task but now I cannot get into the WordPress back-end. I keep getting the “Unknown Username” error even though I have gone into phpMyAdmin and changed both the username and password multiple times.
Some relevant info: I have had to disable the AIO Security & Firewall plugin or else I get the “Not Available” error. I have had to change domain names for testing purposes if that makes any difference.

Thanks in advance.