Number of Bally MPU LED Flashes, what they mean, and what is wrong if not completing individual flashes are well described on the internet now (partly covered in the manual).
Two examples here:
http://techniek.flipperwinkel.nl/ballyss/rep/index2.htm#flash
https://www.flippers.com/bally-tips.html
Wrong number of flashes and not booting are often due to battery corrosion damage on the board, an intermittent chip socket, or a bad chip - some of this can be repaired, some easily, and other issues need someone with proper expertise.
Make sure you count the flashes correctly.
If it were 6 flashes instead for instance, this would mean that the +43V solenoid voltage is not detected by the MPU - often due to a burned 1A slow blow fuse under the playfield, a poor/weakened fuse holder, or old fuse that simply gave up. More often caused by a locked ON coil, due to a shorted drive transistor on the Solenoid Driver board (and often burned coil if the machine was overfused).
Good luck!
-Sylvain.