"I did not understand the text he sent me."
Translation:Je n'ai pas compris le texto qu'il m'a envoyé.
le texte (French), not le texto (Spanish)
Texto is also a French word: it's an informal word for "SMS text message".
It comes from the word texte + informal diminutive suffix -o rather than Spanish.
Why not accept the formal word 'texte', which is the word given in all the dictionaries I've checked so far?
I think it is because you would assume "texte" is referring to a body of text whereas "texto" is a text message, and because in the sentence it was sent, you would assume it is a text message, but I agree both should be accepted because it has no context so could be either.