This is not an error but a recommendation about the required size of the page metadata image accordingly to the recent Google update, learn more here: https://searchengineland.com/google-now-wants-larger-images-amp-articles-291493. See PDF docs at the paragraph 'Metadata' to learn more about the management of the metadata image for AMP pages.
Now Google wants the article image specified in the page metadata to be at least 1200px wide.
If you received this warning by the Google Search Console, ensure to have updated the JAmp plugin to the version 1.7.3 or later so that JAmp will generate and resize images accordingly to the latest Google requirements, changelog: https://storejextensions.org/forum/view/28-changelogs/7926-re-jamp-changelog.html
After that you have updated the JAmp plugin and checked that images are resized to the new dimensions, you can request a validation fix in the Google Search Console. After a few days if everything is ok, the validation will be marked as passed successfully.
As an alternative, obviously you can just replace images used for the page meta image with larger ones to be compliant with required dimensions. In such case there are no actions needed by the JAmp plugin to resize images having already correct dimensions and therefore left unchanged.
Notice that the Google Search Console doesn't work very well during this process and often generates false positives failing the validation fix even because of the Google cache usage. You may need to change the name of the AMP metadata image in order to have a correct validation fix.
Why the Google Search Console warns about 'Image size smaller than recommended size'?