"I saw you months later."
Translation:Te vi meses después.
Yo vi meses después = I saw months later. You just left out the word for you. See your other question.
You can start your sentence with yo (it's not needed here, but it's not wrong to use it). But right before the conjugated verb, you need a pronoun for you.
Yo te vi meses después. I saw you (tú) ...
Yo lo vi ... I saw you (usted masculine) ...
Yo la vi ... I saw you (usted feminine) ...
Yo los vi ... I saw you (ustedes masculine) ...
Yo las vi ... I saw you (ustedes feminine) ...
These are some of the pronouns for you that Duo should accept.