"My heart is not a football."
Translation:Moje srdce není míč na fotbal.
"Fotbalový míč" was missing, so that explains part of your problem.
The suggested translation Duo offers when you make a mistake (or the answer is missing as it was here) is not necessarily the 'main' translation but one of the possible translations that Duo believes is the closest to your attempt. If no available translation seems to be close enough, it offers the 'main' translation.