No, the GoPro Hero12 Black is not compatible with the Lexar CFexpress Type B Professional Gold 128GB.
GoPro HERO12 Black has a microSD-only card slot (no CFexpress support of any type). Its own product page confirms only microSD card compatibility -- CFexpress Type B uses a physically different, much larger connector and cannot be inserted into a microSD slot.
What works instead
See all compatible camera for the GoPro Hero12 Black →“Memory Storage microSD 1x microSD™ rated class A2 V30 or higher”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)