"I have a cat and a dog."
Translation:Ho un gatto e un cane.
While that is the classical recommendation, that's never been a strict rule, and varies regionally. If e is followed by a vowel and there is no pause between them (comma, point and so on) you can use ed; if duolingo doesn't accept it you should report it as a mistake on their part. Personally, I mostly use ed in "ed è", but it's pretty much personal preference.
I come to the comments section in hope to understand more about "e" and "ed".
There are WAY TOO MUCH noise on "gatta" and "cagna"!!
If you know the rhyme and reason for "un gatto e un cane" instead of "un gatto ed un cane", pray tell!
This is an exercise to understand "Conjunctions".