"I have to eliminate it."
Translation:Lo tengo que eliminar.
What is wrong with the translation "Lo necesito que eliminar"?
That would translate to "I need to do it", which (in British English) is the same as saying "I have to do it".
Lo (or la) is used for direct objects (the noun that receives the action of the verb, in this case "it"). Le is used for indirect objects (the noun that receives the direct object - this sentence does not have an indirect object, but they're often a person or thing that has something happening to it). Note that lo/la are engendered, but le is used for indirect objects regardless of gender.
The object pronouns generally go before the predicate - usually the conjugated verb (tengo) - not the infinitive (eliminar). Unless, of course, you add them to the end of the infinitive, but Duo doesn't seem to like that.
"When you don't know what to do with a pronoun, put it in front" seems to work well in duo Spanish.