Machar nitpalel be'kotel ha-ma'aravi.
The right answer should be: tomorrow we will pray at the western wall
The only difference I see with your answer is that you haven't capitalised "Western Wall", and as a place name it should be capitalised. However, Duolingo doesn't care about upper and lower case when it marks answers anyway.
Can you also just say בכותל?
Do you use כֹּ֫תֶל exclusively for the Western wall, because usually a wall is called חוֹמָה?
Usually a wall is called קיר Kir.
You're right that Kotel כותל is used exclusively for the Western Wall.
Choma has a different meaning - a large (or long) dividing wall used for bordering and protection against hostile nations...
Any dividing wall, it seems, can be a חומה (I've been checking the translations at Reverso Context).
The wall of a building (inner or outer) is a קיר.
In technical vocabulary, especially biology (cell wall) or geology (cave wall), דופן is used.
Wailing Wall is in common usage in English.
DL accepts Wailing Wall.
I stupidly translated it very woodenly and said "in". I think that's literally a defensible translation though
Can anyone explain why it is ב?
ב is “at” here, as you can see in the translation. Prepositions are notorious for not translating exactly the same across languages.