There are a number of reasons why you might not be able to open a PNG file in Photoshop. The most common reason is that the file has been corrupted, either during transfer or while it was being saved. If the file is corrupted, it will not be able to be opened by any program, including Photoshop.
Another possibility is that the file is not actually a PNG file. While the file extension might be “.png”, it is possible that the file has been renamed or saved as the wrong file type. If you try to open a JPEG file in Photoshop, for example, you will receive an error message.
Finally, it is also possible that your version of Photoshop does not support PNG files. Older versions of Photoshop (prior to CS6) do not support PNG transparency, for example. If you are trying to open a PNG file in an older version of Photoshop, you will need to upgrade to a newer version in order to view the file correctly.