And the same issue again: You are unable to explain why.
You read the log?
Probably you have inssufficent free space…
And read my old 2017 post for the instructions, are still valids for 6.49.7 and for 7.6:
The RouterOS and the current RouterBOOT must be the same (6.49.7 or 7.6) before use the factory boot update (6.49.7 or 7.6)
http://forum.mikrotik.com/t/protected-routerboot/85672/1