Why user is null?

[toxic ridge]

All the user IDs my bot works with are from users that joined my server.

[unique mist]

Can you share more of the code? That one statement isn't helpful to go off

[toxic ridge]

        string messageToSend = String.Empty;
        ulong guildId = (ulong) command.GuildId;
        
        foreach (var user in userDb)
        {
            var guildUser = _client.GetGuild(guildId).GetUser(user.Id);

            if (guildUser != null)
                messageToSend += $"*{guildUser}* {RankGrade.StringCreator(user.RankGrade)}\n";
        }

        await command.RespondAsync(messageToSend);

[unique mist]

Only thing I can think is that you have null entries in userDb

[toxic ridge]

No, there are no null entries.

Perhaps this is due to the way Discord works, if the user writes a command or participates in correspondence, then the bot receives user instance.

You can see on a screenshot that bot doesn't get null after I get user info.