"I was very tired yesterday."
Verbs that describe a mental or physical state of being are used differently than regular verbs. victoriawl93 is correct that you use つかれました (tsukaremashita) in the present when you are currently tired.
'-mashita' or ta-form is generally used when you want to describe a physical / mental condition.
I feel ____ (at that moment)
___ appears to be .... (at that moment)
e.g., I'm so tired. (to be tired ... tsukare-masu)
(polite form ) tsukare-mashita 疲れました。 (past tense of 'masu-form')
(friendly form) tsukareta 疲れた。(ta-form)
The past tense shows that you got tired and are now in a current state of being tired. If you want to talk about being tired in the past, you use the past continuous to show that you got tired at some point in the past and continued to be tired in the past (but now it's over).
つかる or つかります means to tire in the present or the future. (Present Tense) つかれった or つかれました means to tire in the past. (Past Tense) つかれっています means to be in in the process of tiring. (Progressive Tense) つかれっていました means to have been in the process of tiring. (Past Progressive Tense)
So I was just asking if anyone knew why the past progressive つかれっていました would be used in this translation instead of the simple past tense. I suspect that it is just because つかる is just a verb that is habitually used in the progressive form, however I was hoping someone could confirm that.
Why is kinou the subject, with a particle? This sounds to me like "Yesterday was very tired." A lot of times theres no particle after the "temporal phrase" (like kinou and ashita), so why is there one here? (Of course it does makes sense for a sentence like "kinou ha nichiyoubi")
For context, I entered "kinou(,) totemo tsukareteimashita" and it was wrong
は is a topic marker, not a subject marker. It's not necessary after such relative time references, but it can be used to emphasize 昨日（きのう） as the topic of the sentence.
That said, the sentence you wrote (if you wrote it properly in hiragana or using the relevant kanji) should have been accepted.
The answer it gives you as correct isn't necessarily the answer it was expecting you to build, but rather the answer that your "incorrect" answer was closest to. It can be confusing, but when you can't build the "correct" answer they give you, it's best to check the sentence discussion and see what the answer is at the top.