[Drunken F00l]

To start off this thread, I would just like to thank Drunken F00l for creating SourceOP.

You're welcome.

Whenever they type the player commands (such as !allguns), it never succesfully gives them allguns.

!allguns is not a SourceOP command. Perhaps they are trying commands that aren't SourceOP commands? What other commands don't work?
See this page for a list of player features and commands (the player ability pages are not done, but you get the idea):
http://www.sourceop.com/wiki/Player_Commands

And some odd reason when I start my server it says that I've got 1452 credits, but as soon as I make my server nonlan by typing "sv_lan 0", it says I have 0 credits

This is because your SteamID on a LAN game and non-LAN game is different. Credits are based on a player's SteamID so there is no way around it.

and I can't get anymore by killing NPC. It says something like, "You've reached the max credits on this map."

You're interested in the CVAR DF_credits_maxpermap. Set it to something high (like 999999).