Unable to select variation as a target for Best Bets

Does anyone know why when creating a Best Bet and selecting Commerce content only categories and products can be selected? When I select a variation, the OK button is disabled.

We are searching for variations so Best Bets should be applied for those. When selecting a product, we do not get search results (as we are searching for variation).

Here is the code for the search:

var request = _client.Search<VariationContent>(GetLanguage())
                .For(query)
                .InFields(
                    x => x.Code,
                    x => x.DisplayName,
                    x => x.DisplayDescription
                    )
                .UsingSynonyms()
                .UsingAutoBoost()
                .ApplyBestBets()
                .Skip((page - 1) * pageSize)
                .Take(pageSize)
                .FilterForVisitor();

And here is the screenshot of the best bets target selection when a variation is selected - notice "OK" button is disabled:

And according to documentation, it should be possible:

https://webhelp.episerver.com/latest/en/find/best-bets.htm