"Marxism (or any other such system) has never been tried" is such a tired and pointless argument.

There have clearly been repeated attempts to implement it. If you say that none of them have actually implemented it, you should consider that strong evidence that it cannot actually be implemented. These attempts have clearly universally led to great harm, which is evidence that the attempt inevitably leads in that direction.

If you expect to defend Marxism this way from the accusation of having "failed", then at best you are kicking the ideological can down the road, and at worst engaging in No True Scotsman fallacy.