It's been a long while for me with SNMP, but are you perhaps trying to access something in a private versus public MIB and this is why it wants a username? And I'm not even sure if the difference of private vs. public mattered there as opposed to the simple fact that the private MIB is just proprietary. I think some more details about what you're doing, what version of net-snmp you have, and a cut/paste of your command attempt and error might be helpful.
After a brief search:
https://www.paessler.com/manuals/prt...ommunitystring it would be the Community String concept, but that's for v1/v2 and it says for v3 they went to username/password. So you need to know that information to be able to access the objects on that particular tree.