You can use the speedbrake as normal during flight but you can not arm it, so you will not have automatic speedbrake deployment on touchdown.
"The SPEED BRAKE DO NOT ARM light comes on when the
automatic operation of the speedbrake system does not arm
correctly. This occurs when the speedbrake lever is in the
ARMED detent or either thrust lever is in reverse and one of
these occurs:
#Antiskid inoperative
#Autospeedbrake actuator not retracted
#Disagreement between wheel spin speeds and air/
ground signals
#Internal faults in the autospeedbrake module."
Sounds to me like it simply won't operate
automatically.