You can try 2 things.
If the filename is very long or has non-ascii characters (ä,ë, ø, … , special characters, chinese, japanese or other international characters), rename the file to something short.
This was a bug in a certain version in UE4, i remember.
Import failed via drag n drop to the editor, but was fine with the import-button in the content browser.
When i experienced this once, i could solve it by opening the png with an external image editor and exported a new file, which was ok for UE4. Perhaps something in that png was not within specs.
(It was Affinity Designer in that case, but Photoshop might do the same.)