Why can't I save an image as PNG in Photoshop?

Related questions

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2. Which is better PNG or TIFF?
3. What is the best photo format for printing?
4. Is PNG better than JPEG for printing?
5. Can a PNG be CMYK?
6. Is PNG lossy?
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10. What is PNG format of picture?

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19. #option
20. #should
21. #choose