preferably

[lottafran]

I know the usage of the adjective "preferably" in the following sentence is wrong, but I cannot find a good explanation of why:

Meanwhile, others preferably have off-screen conversations.

Could anyone offer an explanation, please?

/Lotta

[Alan]

'Preferably' (an adverb, not an adjective!) normally occurs in non-verbal constructions, e.g.

Let's eat out, preferably at a cheap restaurant.