Hi
According to google documentation: [GA4] Default channel group - Analytics Help
Unassign traffic means that utm tags were there, but they don’t fall into any of the categories. I would recommend checking your utm tags in your ad campaigns first.
A second reason could be a double redirect. That is, the user comes to your site from advertising and he has these parameters in url, but then for some reason he is automatically redirected to another page before GA4 has time to work and all utm tags are lost.
This often happens with Shopify shops, for example, when the ad uses a link to a variant of a product that no longer exists or its id has been changed.
Hope this helps