It's an API of Zombie Plague 5.0. You got the error because it's a third party plugin, it's not included in default amxx package.
Download the zp_plugin_50.zip, compile the cs_maxspeed_api and enable it in plugins.ini. After that, include the cs_maxspeed_api in your plugin and set user max speed to 1.0 (cs_set_player_maxspeed(player, 1.0)). You should call it only once, not in client_PreThink.
https://forums.alliedmods.net/showthread.php?t=72505
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