What is the meaning of farce in the below text ?

I believe the author is referring to the theatrical genre.

https://dictionary.cambridge.org/dictionary/english/farce#

sense 1

Something grotesquely comical.

It means "comical" or "funny."

He meant something like, "The first time history repeats itself, it's a tragedy (sad). After that, it becomes funny."

In this context it means, 'painfully ridiculous' in a tragi-comical way.

In the context you've provided, "farce" is used to describe a situation that has become absurd or ridiculous, particularly in comparison to a more serious or tragic precedent. Marx is drawing a distinction between two phases of historical events. The first phase is characterized by genuine tragedy, where significant and often somber events unfold. The second phase, however, is marked by farce, indicating a repetition that diminishes the gravity of the original events, making them seem ridiculous or ludicrous by comparison.

When Marx says that history repeats itself first as tragedy and then as farce, he is suggesting that the initial occurrence of an event might be serious and impactful, but its repetition tends to be a degraded, less serious imitation. In the case of Louis-Napoléon Bonaparte declaring himself Emperor, Marx views this as a farcical repetition of the earlier, more serious rise of his uncle, Napoleon Bonaparte. The earlier event was a significant and impactful historical tragedy, while the later event, in Marx's view, is a grotesque and mediocre imitation that lacks the gravitas of the original, thus rendering it a farce.