Photoshop will not let you save your image as a PNG if you are working in CMYK color mode, so you'll need to switch it to RGB.
To do this, make a copy of your image and open a new document. In the settings for your new document go to Color Mode and make sure RGB color is selected. Next to color mode is another dropdown. Make sure you have either 8-bit or 16-bit selected in this dropdown, as PNG files only support 8-bit or 16-bit RGB.
Then, paste your image in the new document. Try "save as," at this point, and if it lets you save as a PNG you know that was the problem.
Instead of saving, you could try to export your image file as a PNG. Go to File --> Export --> Export As, and select PNG.
This is the trick I normally use. If you hold down the alt/option key while you click "Save As," it removes any non-PNG allowed formatting and saves a copy of your image that you can work with.