This sounds like you are trying to open an effect made with popcornfx version 1 with the popcornfx editor version 2 (we released version 2 in 2019, all previous versions are version 1)
You can get the latest v1 editor here: https://wiki.popcornfx.com/index.php?title=PK-Editor_v1.13
.pkfx files are the raw effect files. .pkkg (package files) are basically zip files containing effects and the other resources they need (textures, meshes, audio files, etc)
So if you only have a .pkfx file, you'll be able to open the effect, but it might be missing some textures.
Once you have downloaded the v1 editor, create a new project and copy/paste the .pkfx file inside, it should then appear in the editor content browser.
Hope this helps.