"I tell her that I love him."
अनुवाद:मैं उसे बताती हूँ कि मैं उससे प्यार करती हूँ।
Both your sentences are correct:
- मैं उसे बताता हूँ कि मैं उससे प्यार करता हूँ - spoken by a male
- मैं उसे बताती हूँ कि मैं उससे प्यार करती हूँ - spoken by a female
Duolingo accepts both of these as correct answers, but it can only give one as the "recommended" answer. For fairness and equality the Hindi team alternative between giving the male and female version as the "recommended" answer - but both are accepted.