"There is a mistake in your bill."
Here's why:
* "There" is an expletive, a word that fills a grammatical position but has no real meaning. It's used to introduce the real subject of the sentence, which is "a mistake."
* The verb "is" agrees with the singular subject "a mistake."
* "In your bill" is a prepositional phrase that modifies the noun "mistake," indicating where the mistake is located.
"On bill" is grammatically incorrect because it lacks a definite article ("the") before "bill" and doesn't make sense in this context.