The 404 error means the link of the item was not found. Do these portfolio items exist? If yes, go to the single-portfolio-7, portfolio-5 and side-image-3 and delete them if you do not use them.
Clear cache and refresh your portfolio pages.
The portfolio files are all necessary for the theme. You can not delete them. The mentioned portfolios are ITEMS, not files. If you do not have such portfolio items in your site, then i recommend you clear cache first. Or perform a database optimization operation. Clear up all junk content.
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