Am Oktoberfest kann mann viel Bier trinken.

"Oktoberfest" and "Bier" are nouns, so they are written with capital letters.

There is two words that are quite similar. "Mann" for man, so for a male person. "man" means "someone"/"you"/"one". In the sense of how you wrote it "you can dink a lot of beer". But thatone is written with just one m.

"Getränk" is also a noun and means "drink". "to drink" as a verb is "trinken" in German.

And it is "am Oktoberfest" in the sens of "at the", not "in".