No, the Canon EOS R8 is not compatible with the Lexar CFexpress Type B Professional Gold 128GB.
Canon EOS R8 has a single SD UHS-II card slot and no CFexpress slot of any type. Canon's own manual lists only SD/SDHC/SDXC (UHS-I, UHS-II) as compatible card types — CFexpress Type B uses a physically different connector and cannot be inserted into an SD slot.
What works instead
See all compatible camera for the Canon EOS R8 →“SD/SDHC/SDXC memory cards UHS-II and UHS-I cards compatible”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)