Warning: Found combinational loop of 1 nodes

You have designed Y as a number of latches, that keep their previous content, when the respective output isn't selected. (Don't know if intententionally, but you did).

Latches are implemented by combinational logic loops. The Quartus design compiler suspects combinational loops as possible unwanted design implementation and thus releases a warning. If the latches have been created intentionally, just ignore the warning or disable the warning level.

You should consider, however, that the unselected outputs must be expected to change in an unpredictable way during the transitions of the select input and effectively latch accidental states, so you may want to change the design to get rid of the latching behaviour.