Not all equipment requires to be earthed, check to see if it has a small square within a larger square on the nameplate, if it has then no earth is required.

If you think about it a lot of keyboards use a wall wart, and the earth pin is normally just plastic, also although it might have a 3 pin plug, if you look at the socket on the piece of equipment it is not unusual to find the centre earth pin missing.

Finally all keyboards have to comply with all local electrical regulations, therefore if premises don’t have 3 pin sockets (And is the norm) then the keyboard will have been designed to take this into account.

Hope this helps

Bill