Do not download the executable alone. You need the complete package.
The base.pbp file is a core component required by the conversion tool. It essentially acts as a "template" or "container" that the software uses to package PlayStation 1 (PSX) disc images into a format the PlayStation Portable (PSP) can read. 2. Why is it missing from downloads? psx2psp base.pbp
Are you converting a or a single-disc title? I can help with the specific settings for those. Do not download the executable alone