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.
请问command跟must中间的主词被省略到哪去了?还是说这纯粹是书写错误?

command在这里是名词吧....

是，是名词(受词)所以我好奇它的主词怎么消失了?

command (主词) must accept and check (动词)