Is citizen a plural possessive noun?

No, the noun 'citizen' is a singular noun, a Word for one person.

The plural form is citizens, a Word for two or more of people.
The plural possessive form is citizens'.
A possessive noun indicates that something in the sentence belongs to that noun.
A plural noun that ends with an s forms its possessive by adding an apostrophe (') to the end of the Word.

A noun that does not end with an s forms its possessive by adding an apostrophe s ('s) to the end of the Word.

Example sentences:

A citizen must register to vote. (singular)

The mayor responded to the citizen's letter. (singular possessive)

Hundreds of citizens lined the street to watch the parade. (plural)

The citizens' committee discussed the proposal. (plural possessive)