Likely the deprecated QT MPEG4 codec. I’m assuming you are on a Mac. PP simply doesn’t play everything. VLC does (almost). You’ll need to convert. Since VLC can play it, you can probably convert with anything that uses FFMPEG for core conversion. You can convert with VLC but it’s a PITA. Presuming you want to convert to a pro format, like ProRes, I suggest FFWorks. In short, it’s a matter of finding an app that can read the file and convert to the format you want. Worst case, you’ll need Windows, Linux, or an old Mac (pre-deprecation) but chances are FFMPEG can handle it.