"A crab and a turtle."

Translation: Ένα καβούρι και μία χελώνα.

[Langenth]

'καβούρι' ends with a vowel but masculine? Is there a gender rule for words that end with a vowel that is not Α (feminine) or Ο, Ε (neuter)?

[D_..]

'καβούρι' ends with a vowel but masculine?

It is not masculine, it is neuter! :)

Is there a gender rule for words that end with a vowel that is not Α (feminine) or Ο, Ε (neuter)?

There is not unfortunately and there is overlap between endings and genders but there are patterns. I suggest looking here http://www.foundalis.com/lan/grknouns.htm to see what the most common patterns are.

[MariaM873341]

Are you?

[inbal966822]

I don't understand when we say "ενα" and when we say "μια". Can someone tell me the difference?

[D_..]

There are three grammatical genders in Greek and the article has a different form for each gender: ένας is masculine, μία is feminine and ένα is neuter. These are all explained in the tips section for the Basics 1 unit at https://www.duolingo.com/skill/el/Basics-1/tips-and-notes.