I understand that in many cases "ein " can have a similar meaning/implication as "eine _" as they both imply a singular case. But it seems a little confusing, and perhaps unfair, to mark "Eine Baum" incorrect if they simply ask the user to translate "A tree". Can someone show me where I might be taking a wrong turn on this thinking?
The use of "ein" or "eine" (both "a(n)") makes the same distinction as "der", "die" or "das" (all "the"). Nouns in German have gender. This isn't the same as human gender, but rather more like a grammatical category of words. Unfortunately, there aren't any rules that govern noun gender, just some general patterns (and a lot of exceptions), but to speak German correctly, you must use the correct noun gender with the correct noun. The three genders are masculine (der, ein), feminine (die, eine), and neuter (das, ein). You really just have to learn the noun's gender when learning the word, as this effects which article (der, die, ein, eine, etc.) you use with the noun, and it also affects how grammatical case affects the word and which adjectival endings you would use. That's a lot to take in for a beginner, so just focus on the fact that nouns have gender for now.
For example, "Mann" ("man") is masculine, as you'd expect, so it's "der Mann/ein Mann". But "Tisch" ("table"), "Strand" ("beach") and "Mensch" ("human") are also masculine.
"Frau" ("woman") is feminine, as you'd expect, so it's "die Frau/eine Frau". But "Küche" ("kitchen"), "Stadt" ("city") and "Einsamkeit" ("loneliness") are also feminine.
"Haus" ("house") is neuter, as you might expect, so it's "das Haus/ein Haus". But "Kind" ("child"), "Gebäude" ("building") and "Buch" ("book") are also neuter.
If you want me to, I could go in to greater detail about the patterns and how noun gender comes into play with the different cases and adjectival endings, but for now just thing of them as categories. In this case, "Baum" is masculine, so it must be "ein Baum". Sure, you would be understood if you said "eine Baum", but it would be immediately noticed as incorrect, because "Baum" is not feminine. It is similar to how in English, saying "an tree" sounds incorrect and unnatural compared to "a tree".
EDIT: Note that "ein" and "eine" have virtually always the same meaning, gender of the following noun is the main difference.