@RolloM welcome to the forums 
It sounds like perhaps the ISO file that you’ve downloaded may have been corrupted in some way. You could verify everything is OK using the instructions on the Linux Mint website here: https://linuxmint.com/verify.php or alternatively simply try downloading it again.
You should not get the message ‘Missing partition table’ from Etcher with a valid ISO, nor should you see the ‘Please select a valid ISO file’ message with the Windows tool.