@ichitaka
According to WIKI:
https://github.com/recalbox/recalbox-os/wiki/Add-system-bios-(EN)
Your MEDNAFEN_PSX BIOS does not have the correct names, does it? (The correct names and extensions are in lowercase in this case, different from the main BIOS of playstation, which is all in Uppercase)
924e392ed05558ffdb115408c263dccf SCPH1001.BIN MEDNAFEN_PSX (X86/X86_64) :
8dd7d5296a650fac7319bce665a6a53c scph5500.bin
490f666e1afb15b7362b406ed1cea246 scph5501.bin
32736f17079d0b2b7024407c39bd3050 scph5502.bin