Server Access "Username not found!"

I get "Username not found!" when I try to give server access to an Aternos community member DaFlatJack, I must be missing something easy...?

Quote from PluginsMaster

They could have changed their username or closed their account. See if you can ask them if they changed it.

I'll ask if they did change it, but I'm pretty sure they are an Aternos member as they were shown online and I was able to message them to relay the snafu.

Quote from AlexanderSniper1

What do you mean, are you adding their forum username? If so, they need to make an Aternos account. The aternos.org account is not the same as the forums account.

Oh, I didn't realize they were two different entities... I suppose I was under the impression that Aternos board members were also Aternos members... good to know, Thank you!