Struggling, unpublished writers are sure that publishers, who seek out and publish new authors, are nonexistent.

Here is the full question:
Struggling, unpublished writers are sure that publishers, who seek out and publish new authors, are nonexistent.
Find and correct the error in this sentence

The correct sentence should be without the second comma:
Struggling, unpublished writers are sure that publishers who seek out and publish new authors are nonexistent.
 publishers who seek out and publish new authors are nonexistent is a part of one clause that does not possess two different meaning, so it should not be separated by a comma.
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Fixing the errors and resubmitting it