No, the Canon EOS R5 Mark II is not compatible with the Lexar Professional CFexpress Type A Gold 256GB.
Canon EOS R5 Mark II's Slot 1 is CFexpress Type B only. Canon's own compatible-cards page states 'CFexpress cards Type-B compatible' — CFexpress Type A uses a physically different, smaller connector than Type B and cannot be inserted into this slot.
What works instead
See all compatible camera for the Canon EOS R5 Mark II →“CFexpress cards Type-B compatible SD/SDHC/SDXC memory cards UHS-II and UHS-I cards compatible”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)