"Könnt ihr mir mehr darüber sagen?"

[MrMehdi]

Why is "Could you tell me more about that" incorrect?

[thedoctar10]

Could is the subjunctive mood of can, which is different. Ihr könnt vs ihr könnet (see https://en.wiktionary.org/wiki/können#German for the full table)

[izo_sak]

why is it ihr and not Sie? And even so, shouldn't it be capitalized?

[djdejong]

Both mean "you" but differ by formality and can differ by number.

"Sie": nominative, singular/plural, formal 2nd person pronoun

"ihr": nominative, plural, 2nd person pronoun

"ihr" in this context wouldn't be capitalized.

(https://deutsch.lingolia.com/en/grammar/pronouns/personal-pronouns)

[Alicia54203]

I said 'could you talk to me about that more?' and it wasn't accepted - it is a close translation.