Sorry for delay! Meant to reply to this but been kept busy!
There are no distance penalties for a thrust reverser inop on the B757/767. The EICAS message L/R REV ISLN VAL in the air is a fault in the reverser system (Isolation Valve) but still creates no distance penalties and should work normally on touch-down. These EICAS messages on the ground will require the reverser to be locked-out, creating your reverser inop situation!
Cheers
JB