Learn Russian: ЕС не принял наш план.

ЕС stands for Европейский союз (European Union). When dealing with indeclinable acronyms, Russian usually assigns grammatical gender based on the main underlying noun. Since союз (union) is masculine, the acronym ЕС is treated as masculine, which is why the past tense verb takes the masculine form принял.

How is the abbreviation ЕС pronounced when reading this sentence?

It is pronounced by its Russian letter names: Е-эС (ye-es). While some Russian acronyms are read as full words if they form a pronounceable syllable (like МИД or вуз), ЕС is treated as an initialism and read letter by letter.

Shouldn't the direct object of a negated verb be in the genitive case (нашего плана)?

Not necessarily! While the "genitive of negation" is taught at earlier levels, Russian actually allows both accusative and genitive cases for negated objects. The accusative (наш план) is preferred here because the plan is a specific, definite thing that already exists. The genitive is typically used for abstract concepts or when you want to emphasize the complete absence of something.

Is принял only used for "accepting" things like plans or offers?

No, the perfective verb принять is extremely versatile. Aside from accepting or adopting a plan, law, or offer, it is the standard verb used for "taking" medicine (принять таблетку), "taking" a shower (принять душ), or "receiving" visitors and patients.