Hi,
Are you not getting the option to save the file as PNG in the File Type drop down in the Save As dialog box? If yes, please check the image mode for the document by going to the Image menu, then Mode. If you're working with the CMYK document or a 32-bit RGB document, change that to RGB 16-bit or 8-bit to check if you get the option to save the file as PNG.
If you're facing a different issue, if would be helpful if you could elaborate the issue a bit more and also share the version of Photoshop you're using along with your computers operating system.
Regards,
Nikunj