Snapshot - Error posting to API: 500 Internal Server Error

I have 5 images remaining on my site waiting to be smushed, but when I click on Bulk Smush I get this error message:

Error posting to API: 500 Internal Server Error
Error posting to API: 500 Internal Server Error
Error posting to API: 500 Internal Server Error
Could not find /var/www/html/wp-content/uploads/2016/07/02***.jpg
Could not find /var/www/html/wp-content/uploads/2016/07/sh***.jpg