IPlayer is an object that is connected to all player objects (BasePlayer), at least if they are actual players and not NPCs. But, even for NPCs it would not throw a compiler error but instead might show a different error altogether if not considered as a possibility in the plugin. For it to show this error seems to imply that the game libraries don't know about that object, which is pretty weird.
The best advice I can give is to validate your server files and reinstall oxide.