Does anyone kno where should I add the comma at?.

Only one comma after "21"

Explanation:

Remove the comma after "March"; add a comma after "21." This is because "but" is a conjunction and it flows better with a comma preceding it.

the sentence does not violate the phrase structure second rule (introduced by noam chomsky) because noun phrases consist of a determiner such as a, the or every, and a noun. in the sentence, the noun phrase the cake includes a determiner (the) and a noun (cake).

on the other hand, the first rule indicates that a sentence encloses a noun phrase, followed by a verb phrase. the example sentence can be analysed as follows:

john (n) + ate (v) + the (det) + cake (n).