No problem, it just helps to know which device we’re talking about so we can give more targetted advice The lower image is a little too small to be readable, so I can’t see in detail the error message, sorry. But I get the idea of what your set-up and issue is.
The name and password should have been set up when you first did the set-up of the unit (at least they were on mine when I did it, although mine is a MyCloud Mirror rather than a single drive MyCloud). Based on that the unit’s name is probably WDMyCloud (mine was WDMyCloudMirror) and the username and password should be either admin and whatever password you set, or whatever user name you set and the password (this is a linux-based device, so remember that the passwords are case-sensitive, if I remember well the user names are automagically converted into all lower case anyway).
What you can do is to reset the device (as ERMorel suggested originally), which will put the admin username/password and the IP address settings back to their factory defaults (but won’t touch your data on the drive). See page 109 of the MyCloud Manual for details, but basically with power on press the little recessed reset button on the back of the drive (just above the USB port) with a pen tip or paper clip for 4 seconds and the device should reboot and reset (which takes a few minutes). Alternatively do it from powered off, press the reset button and hold it for 40 seconds whilst you turn the power on. On my MCM the admin account was “admin” without any password, so yours may well reset to something similar (if not then try “admin”/“admin” user/pwd).
To your other question - the guest account will only have access to the public shares on the device. If you have any non-public shares, access to those is of course set up on a per-user basis and the guest account won’t have any access. So if you only have public shares then using the guest account won’t matter, but if you want to have any kind of password access control or restriction then you need to be using user accounts. The three in your image above (Public, Smartware and Time Machine) are all public shares when initially set up (although you can make the latter two private shares if you wish by changing their configuration via the dashboard).
What I did on mine is to leave the admin account completely alone except for giving it a password, and then set up other user accounts for my household with my own personal named account also having admin rights. See section 7 of the manual (page 51 onward) for details about doing that once you get back in again.