"It is forbidden that the citizens wait here."
Translation:Vatandaşların burada beklemeleri yasaktır.
this is how the relative clauses are formed in turkish. the literal translation of the sentence is:
The "waiting here" of the citizens is forbidden.
that's why the word vatandaşlar takes a genitive ending.
In Turkish, gerund phrases tend to get used a lot more than in English. It is similar to literally saying "The citizens' waiting here is forbidden"
Is it required to say "beklemeleri", or does "beklemesi" work too without the plural ending?
Nope..."burada beklemeleri" is a phrase that cannot be broken up.
I wrote 'vatadaşların burada beklemeyi yasaktır' - it corrected it to 'beklemesi'. Why is 'beklemeyi' wrong?
You do not need (and should not) to take the accusative here, as the gerund is nominative.
It is a genitive / possessive construction. Literally you could ask something like: Whose waiting is forbidden?