Learn Russian: У нас было полтора литра сока.

The word полтора is used with masculine and neuter nouns, while полторы is strictly for feminine nouns. Because литр (liter) is a masculine noun, it takes полтора.

The English translation says liters in the plural, so why is литра in the genitive singular?

In the nominative and inanimate accusative cases, полтора plays by the same rules as the numbers 2, 3, and 4. It requires the noun that directly follows it to be in the genitive singular, turning литр into литра.

Why is the verb было in the neuter singular form rather than plural?

When a sentence's subject is a number or a quantity phrase like полтора литра, the past tense verb usually takes the neuter singular form. The grammar treats the entire quantity block as one single, neuter concept.

Why is сок (juice) also in the genitive case as сока?

This works exactly like saying a liter of juice in English. We use the genitive case to link a unit of measurement to the actual substance being measured, so сок becomes сока.