原文:A slave device that implements Packet Error Checking must be prepared to
receive and transmit data with or without a PEC.
During a slave receive transfer, after the device has identified the protocol
and command must accept and check the additional PEC for validity.
請問command跟must中間的主詞被省略到哪去了?還是說這純粹是書寫錯誤?

command在這裡是名詞吧....

是，是名詞(受詞)所以我好奇它的主詞怎麼消失了?

command (主詞) must accept and check (動詞)