Couple of things. Which RB is it exactly? RB? RB Pro-S? Pro-SD?
Which back is it exactly? RB, Pro-S? Pro-SD?
Is the rotating back in the right position? Either vertical or horizontal, not half way between?
The dark slide is OUT, right?
Where is the lever right under the winding lever?? Red dot showing or not?
You have the lens OFF?
Your back is on or off?
The mirror is all the way DOWN?
I'm having difficulty determining what you have right now and where everything is.
There is no such thing as mirror lock but there is an Interlock if you have Pro-S or later.