The usage of 该 in sentences
就该吃饭了 is missing a subject for example 你
=> 你 (現在就) 该吃饭了 You should be eating now

就吃饭了 is not grammatically correct, it's missing 要 and a subject
=> 我 (就) 要 吃饭了 I 'm going to eat

该吃饭了 is proper because it implies the subject to be I/You
该吃饭了 means I/You should be eating now, or Time to eat.