client.player is only available serverside if native MP mode is disabled
Setting the position will also not work serverside because a client is actively syncing the element (the element in your case is a player ped). The server's position is reset when the next sync packet arrives from it's syncer. You will need to use a network event to tell the syncer client to set the position.
When using native MP mode, all elements are handled clientside. The server's only role in that mode is to broker connections. It has literally NO awareness of any elements in the game. If you need to use serverside elements, disable native MP mode.
Setting the position will also not work serverside because a client is actively syncing the element (the element in your case is a player ped). The server's position is reset when the next sync packet arrives from it's syncer. You will need to use a network event to tell the syncer client to set the position.
When using native MP mode, all elements are handled clientside. The server's only role in that mode is to broker connections. It has literally NO awareness of any elements in the game. If you need to use serverside elements, disable native MP mode.