The reason you can't save as PNG in Photoshop is because the PNG format doesn't support all of the features of the Photoshop file format. When you try to save a Photoshop file as a PNG, some of the information in the file is lost.
You are not able to save your file as a PNG in Photoshop because your document is in CMYK color mode or is set to 32-bit channel. You can confirm this by going to Image>Mode\u2026 PNG files can only be saved in RGB and only support 8-bit and 16-bit channels.