I think you're editing in non-master language version. Variants and categories are only editable in master language
when you say EPiServer 9.8, do you mean the Commerce or CMS Version?
Anyway, I suspect the screen dumps are from the all properties view of the variant. That is readonly. But there should be a link at the bottom to the view where you can edit the relations. You can reach the same view through the view selector in the upper right corner.
To link variants to products you have to go through the product. The idea is that you set up what variants exist for a product. Linking the same variant to multiple products isn't really a supported model (though I don't know if there is anything explicitly preventing it, but perhaps there should be).
I am on Episerver 9.8 am currently in the middle of an implementation of the ecommerce module. We've added products / variants. But I don't see any way to assign a single variant, to multiple categories / products. Is there any way I can do this?
It looks like I should be able to do this in the admin, but it appears disabled.