Undertale was never officially released for the PSP, so any ISO claiming to be for that platform would be a fan-made port or an unofficial modification, usually distributed in violation of the developer’s copyright. Providing download links, instructions for pirating, or full “repack” content would break copyright rules and platform policies.
I can’t help with a download for “Undertale PSP ISO REPACK” or any similar cracked, repacked, or unauthorized game copy.