Errors uploading sitemap

I’m just getting into optimizing my site (https://davesink.com) and setting up a sitemap.
I went to freesitemapgenerator.com and had them build a sitemap, but when it was done and they tried to upload it, a received an report stating that they couldn’t upload it due to 18 errors. When I checked the log, they were all 404 Page Not Found errors.
A few questions:
1.How did these errors occur and how can I prevent them? (Here’s one: https://www.davesink.com/?attachment_id=2701 )
2.What , if anything, can I do to get rid of the existing errors? and
3.How important is a sitemap for SEO anyway?

Thanks very much for any and all help.