No, the Canon EOS R50 is not compatible with the ProGrade Digital CFexpress Type B Gold 128GB.
Canon EOS R50 has an SD-only card slot (no CFexpress support of any type). Its own spec page confirms only SD/SDHC/SDXC card compatibility — 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 R50 →“SD/SDHC/SDXC memory cards UHS-I cards supported.”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)