The first 3 are ascending beeps are telling you the ESC is powered on and the cell count of the lipo is being detected. The 2 bleeps after that is telling you that the FC is ready. So if you only hear 3 beeps followed by 1 single beep lt points in the direction of a communication error between ESC and FC.
Things to check are:
Wiring between ESC and FC. It can be loose or even broken. Or you did not seat it properly and bent a pin...
Verify betaflight firmware (e.f. flashed incorrect target)
Connect the FC to your PC and check if it boots up and is therefore recognized by betaflight.
Also worth checking is the motors tab in betaflight to verify that everything is correctly set up.