Apple - "The document could not be saved" - Preview error

I was facing the same issue, I finally got the issue resolved by following steps:

1. Go to File
2. Click option 'Export as pdf'
3. Save the file name with pdf extension (like 'file_name.pdf')

This worked for me, so give it a try.

I realised this is a bug in the Mask tool. Remove the mask and the image saves without any problem. Avoid this tool until Apple addresses the issue. Checked on High Sierra 10.13.4.

A solution to this is to take a screenshot of the image preview before saving it(solution from @gpbl).