Unable to format sandisk ultra 16gb usb3.0 pendrive in mac

Your USB stick may be defective.

How do I fix a Write Protect error?

Write protection errors occur when a flash drive detects a potential fault within itself. The drive will go into write-protected mode to prevent data loss. There is no method to fix this. To verify the issue is related to the flash drive and not your computer we recommend that you try plugging the USB drive into another port on your computer. You should also try the drive on another computer if possible.

Source: https://kb.sandisk.com/app/answers/detail/a_id/8656/~/write-protect-error-on-usb-flash-drives