Hi Lorenzo.
L9963E does not have any register called "FSMstatus"
FSMstatus is actually the name of a field of the FSM register [address 0x12]

This register contains the current state of the device.
Regarding exiting from Sleep state, take a look to page 15 of the datasheet:

So, for example, two SPI transactions are required to exit the sleep state.
Probably, you "may" be able to read a sleep state if enough SPI signals are sent faster than the Twakeup.
A good solution to keep trace of the state of your device is to create a "current_FSM" variable in your code, mirroring the FSM of your IC.
Obviously, you have to maintain it updated, reading periodically the FSM register. But you can also change it manually when a GO2SLP command is sent.
If you need help on coding your L9963E application, you may download the AutodevKit Studio IDE.
It contains a lot of C examples which triggers every L9963E BMS functionality.
To understand more, take a look to the AEK_BMS_Getting started GUIDE
Best regards,
Simone
AEK_Team