It's a quote from Marx talking about why the Paris Commune adopted different political forms than that of the republicans. Saying "ready-made" in this case would be saying that the form of government of an insurgent working class is essentially different from the typical bourgeois state, and that the working class can't simply try to operate the bourgeois state, ready-made because it already exists. Effectively this means that Marx is clear that in order for the working class to free itself, the bourgeois state must be destroyed and replaced by a government created by the working class, and, judging by the Commune and later experiences, will be constructed amid the struggle to destroy the bourgeois state.

Sorry if that's a lot for an innocuous question. But if you're still curious, for fuller context:

The direct antithesis to the empire was the Commune. The cry of “social republic” ... did but express a vague aspiration after a republic that was not only to supercede the monarchical form of class rule, but class rule itself. The Commune was the positive form of that republic. ...
But the working class cannot simply lay hold of the ready-made state machinery, and wield it for its own purposes.
The centralized state power, with its ubiquitous organs of standing army, police, bureaucracy, clergy, and judicature – organs wrought after the plan of a systematic and hierarchic division of labor – originates from the days of absolute monarchy, serving nascent middle class society as a mighty weapon in its struggle against feudalism. Still, its development remained clogged by all manner of medieval rubbish, seignorial rights, local privileges, municipal and guild monopolies, and provincial constitutions. The gigantic broom of the French Revolution of the 18th century swept away all these relics of bygone times, thus clearing simultaneously the social soil of its last hinderances to the superstructure of the modern state edifice raised under the First Empire, itself the offspring of the coalition wars of old semi-feudal Europe against modern France.
During the subsequent regimes, the government, placed under parliamentary control – that is, under the direct control of the propertied classes – became not only a hotbed of huge national debts and crushing taxes; with its irresistible allurements of place, pelf, and patronage, it became not only the bone of contention between the rival factions and adventurers of the ruling classes; but its political character changed simultaneously with the economic changes of society. At the same pace at which the progress of modern industry developed, widened, intensified the class antagonism between capital and labor, the state power assumed more and more the character of the national power of capital over labor, of a public force organized for social enslavement, of an engine of class despotism.
After every revolution marking a progressive phase in the class struggle, the purely repressive character of the state power stands out in bolder and bolder relief...

Later on in the same section Marx describes the different political form of the Commune, mentioning mandated revocable delegation in place of representation, combining executive and legislative functions into the assemblies of delegates, and so on.