"I do not eat meat."
Translation:Я не їм м'ясо.
In most cases (including this one) one can omit pronouns as they can be derived from the verb ending: "їм" can be used only with "я".
However, sometimes it can lead to misunderstanding: "Їсть м'ясо" can mean both "She is eating meat" and "He is eating meat".
Therefore, I would use the pronoun only in cases when omitting it leads to non unique meaning.
In casual conversation, you can omit the pronoun. But it's not grammatically correct, so don't do it on Duo. We do the same thing in English. (Ex: Not going into work today. People will know you mean that it's you who isn't doing that action.) Also, it won't lead to confusion like the user above indicated because it's "я їм" and "вона/він/воно їсть" -- different conjugations. So that isn't the reasons to use the pronoun.