Why is impossible for post-transcriptional editing of mRNA to occur in prokaryotes?

It's impossible because there is no place within the cell for it to occur. In eukaryotes, you have two different regions; the nucleus and the cytoplasm. In prokaryotes, however, you lack the nucleus, meaning you only have the cytoplasm. So, as one can see, there is no separate region for post-transcriptional editing of mRNA to occur in prokaryotes.