when you send this command, you need to place your finger on the sensor. If fingerprint is found in saved fingerprints, sensor should response fp id otherwise it will response with fp not found
if you dont place the finger after this command, sensor should give timeout error. you need to wait a bit
if you don't get any response even after sometime, check your hardware and sensor module's responsiveness by sending some other commands, for example, add fingerprint command.
PS. I have never actually used this or similar sensor. i have been giving you these answers just by reading the datasheet.