Please add some more details about your setup, as detailed in https://retropie.org.uk/forum/topic/3/read-this-first.

What game are you trying to run and what format is your ROM (.cue/ .iso ) ?

The Problem was located:
When using pbp files with sbi files the extension both must be lower or uppercase.
Having them mixed will result in the emulator not finding the sbi file since linux systems are casesensitve.

multidisc PBP Systax is as described in the sourcecode comment above.

Github issue:
https://github.com/libretro/beetle-psx-libretro/issues/241

parasven created this issue in libretro/beetle-psx-libretro closed Playstation .sbi files with PBP or even multidisc PBP #241