Some websites don't work WITH the www. prefix

Your question is based on a somewhat false premise. There is no special reason example.com and www.example.com should go to the same site or both be valid other than that this is what people expect. As far as computers are concerned, they are two completely different names that could go to completely different places. People are free to set them up to go to the same place, different places, or make one work and the other not however they please.

There is simply a general agreement that the host name www will be used for the web site for a domain. But there is no technical reason anybody must follow this agreement.

You question is based on the false premise that example.com and www.example.com are somehow the "same site", one with a prefix and one without. In fact, they are two completely different potential site names that both happen to be under the control of whoever owns or manages example.com.

I found the problem. The administrator of that domain name hasn't set up an address (A or AAAA) record for www.

error@underground ~ $ host itmanagersjournal.com
itmanagersjournal.com has address 216.34.181.202
error@underground ~ $ host www.itmanagersjournal.com
Host www.itmanagersjournal.com not found: 3(NXDOMAIN)

Compare this to what nearly every other site does:

error@underground ~ $ host facebook.com
facebook.com has address 66.220.146.101
facebook.com has IPv6 address 2a03:2880:10:1f02:face:b00c:0:25
error@underground ~ $ host www.facebook.com
www.facebook.com has address 66.220.146.101
www.facebook.com has IPv6 address 2a03:2880:10:1f02:face:b00c:0:25

Unless this is your domain, there's very little you can do to fix it, except perhaps contact the site and advise them of their problem and hope someone there has enough clue to fix it.