"I am going to buy manga in Ikebukuro."
I wrote 池袋でマンガの本を買うつもり。and it wasn't accepted. Can someone explain to me please why is it wrong (if it is wrong)?
You should not end your sentence with 買うつもり, since that is a noun. Your sentence needs a main verb or copula to be grammatically correct.
Also, as a general warning, DuoLingo can be pretty spotty about registering kanji (and vocabulary) it does not teach to you. If you do everything right and it still rejects, that might be the issue.
But in this case, I think you just need to fix the sentence.
I didn’t know it was a known. Duolingo always translates つもり as “going to” if I remember correctly.
You can't really trust English translations, when learning new words in Japanese. Or at least, I would not recommend it. Due to differences in the languages, the translations are usually not literal and a natural-sounding translation may use completely different grammar (or vocabulary) compared with the original Japanese sentence. For example, new students are usually taught that 私は日本出身です。means "I am from Japan.", but a more literal translation would be something like "As for me, Japan is (my) origin/source." or "I am Japan origin.". But neither of these sentences sounds that good in English so you get "I am from Japan" or "I come from Japan" instead.
When in doubt, use a Japanese dictionary to check your understanding. I like Jisho. It is available on-line for free and has a kanji lookup tool.
つもり means "plan, intention, belief". When it is attached to a plain form verb, it forms a noun phrase. So つもり means "a plan" and 買う mrans "to buy", so 買うつもり is "a plan to buy".
Because 買うつもり is a noun, it can't finalize your sentence, but it CAN be used as a predicate in a copular sentence ... which is a fancy way to say you need to add です (or だ) to the end of that sentence to make it grammatically correct.
"It is my intention to shop for manga in Ikebukura."
One more thing ... you can use this sentence pattern to describe your own intentions, but if you want to talk about someone else's plans, you would use a different construct. In Japanese, there are frequently differences when describing the speaker's thoughts, feelings or beliefs, compared with discussing a third party's thoughts/feelings, because you cannot have first-person knowledge of another person's thought processes.
Is the の本 really necessary? I missed the question because I didn't have this.
No, because the verb in Japanese is "to buy". You are buying manga AT Ikebukuro. Rather than going TO Ikebukuro to buy manga. The English translation is a little misleading.
"I will buy manga at/in Ikebukura."
"I will go to Ikebukuro to buy manga."