Sites that require usernames and passwords use what are referred to as “session cookies.” It’s a small bit of information retained by the browser which tells the site “I have logged in.”
Basically, that’s how a browser and a website that accepts logins remembers that the person using it has logged it. The cookie expires when someone closes the browser.
Sometimes people updated their browsers and set the privacy settings to not accept cookies of any kind. There is nothing inherently wrong with that, but it is really overdoing it. It limits a person to essentially window shopping. Without cookies on a site which has user login capability, the viewer can look, but cannot interact. They can disable third-party cookies but not first-party cookies.
All a visitor needs to do is enable cookies. It is a BROWSER function, not a computer function. The method is determined by the browser settings.
For example, for Firefox: https://support.mozilla.org/en-US/kb/fix-login-issues-on-websites-require-passwords
Safari: https://support.apple.com/kb/ph21411?locale=en_US (Note the explanation of third v first party)