Translation:Sorry, I do not have time, I cannot go.
Because another way of saying: “I can’t go” (to a party or a gathering, even a conference) in English is: “I can’t make it” (to the party/gathering/conference). The latter is more often used as part of an apology. It implies that you want to and put some effort in but just can’t manage to make the timing work etc.
Yes, in other exercises Duo even uses "so" to join the two clauses together in English where the Chinese just uses a comma splice, so they really shouldn't mark you wrong if they do it themselves. However some other posters here have been rejected for saying "don't" and "can't" so that could be the reason you were marked wrong rather than the "so".
Yes the comma splice is commonly used in Chinese apparently. Here is a brief Wikipedia extract:
The comma is used to join together clauses that deal with a certain topic or line of thinking. As such, what would appear to an English speaker to be a comma splice is very commonly seen in Chinese writing. Often, the entirety of a long paragraph can consist of clauses joined by commas, with the sole period coming only at the end.