Hi,
The error with the RSS/Atom feeds is a bug in HikaShop linked to Joomla 4. Updating your HikaShop to the 4.6.2 will fix the problem.
However, this is not linked to the "Duplicate without user-selected canonical" error. This means that in the category of the page, you didn't enter the main URL you want to be indexed for the category and that the category can be accessed from different URLs.
So for the URLs with these duplicate errors, you need to enter a canonical URL in the settings of the category and ask Google to recrawl the URL.