"srcset" or "src" attribute must be specified:

I had a similar question regarding missing SRC value. No one answered my question either. Still waiting.

Look at the page source for your /amp/ endpoint for one of the posts which have an error. Do the images have a proper “src” for the image, or “srcset” for new style multi-resolution image groups in WordPress ?

Find the image which has the error, and examine WHY it is malformed.

Did you use F12/console in Chrome, with the development test tag added to end of URL to find the errors in the post per the AMP website docs ?

Plugin can’t fix your content errors; you need to look at the actual rendered page and examine WHAT the issue is. The console view should help with that.