As far as I know there is NO Keyboard Shortcut to Enable Filter on Google Sheet. There is a little longer route though. Holding Control + Option + d would open the drop down of Data Menu. Press f as shortcut key to select the option TURN ON FILTER. This would turn on the Filter. To access your Filter from Drop Down List in a Cell Hold Control + Command + r I have attached a screenshot for other shortcuts pertaining to Data. Hope this helps. -)